So, this is going to be a breakdown of basically all of the guns. This is really for me to help contextualize the current gamestate, but if you're interested, feel free to read. There will be all the maths, so if you don't like math...well, he's kind of ****, so I sympathize, but sometimes he's useful. We're just going to breakdown the non-commander weapons as the Lightsabers are in a class all their own.
Understanding Defense Dice:
As a former and kind-of current 40k player, I contextualize the defense dice in terms of saving throws. White with no surge has 1 good result, so a 6+, white with a surge has 2 results, so a 5+, red with no surge has 3, so a 4+ and Red with a surge has 4 good results, so a 3+. Current breakdown of defense dice:
6+ - AT-RT (he's so lonely here)
5+- T-47, Speeder Bikes, AT-ST, Rebel Troopers (Largest category and most of the vehicles)
4+- Stormtroopers, Luke and Vader (both of the Jedi and the second largest category)
3+ Vader and Luke while using the Deflect ability (smallest category as the only things in it are only in it situationally)
It's important to note here that Legion using more dynamic defenses (based on number of hits) vs a static system (same defenses every time). In X-Wing, Armada, IA and Runewars, you can roll high enough on attack that dodging it is impossible. In Legion, barring the Pierce ability, it is always possible to dodge everything, no matter how unlikely. There's also a bit of an exponential curve over batches of dice, or to put that in plain terms:
The odds of a Stormtrooper who has been hit 3 times rolling all blocks is a 1 in 8. The odds of blocking 4 out of 4 results is a 1 in 16.
Conversely, the odds of a Rebel Trooper blocking 3 of 3 hits is a 1 in 27 (with an 8 in 27 chance of blanking everything BTW), going all the way up to a 1 in 81 for 4.
In short, the difference between a 4+ block dice and a 5+ doesn't seem like much, but you will see WAY more instances of Stormtroopers shrugging off everything than Rebel Troopers.
-Basic Training, Infantry Small Arms-
E-11: White die with a surge
A-280: Black die without a surge
Unarmed: Black die (surge on Stormtroopers, no surge on rebels)
Grenades: Black die (surge on Stormtroopers, no surge on rebels)
Even here, we see a huge difference. While the Stormtrooper's E-11 is only rocking a 37.5% hit chance and the Rebel A-280 has a 50% chance, Stormtroopers have 2 alternate options that buff tier accuracy substantially in melee or grenades that push them up to a 62.5%, where Rebels gain little or no benefit from either weapon over their basic rifle still only having a 50% hit rate.
Factoring in defense dice, a Stormtrooper with an E-11, averages kill .25 Rebel Trooper and a Rebel Trooper averages killing .25 Stormtroopers with their A-280. However, the Stormtrooper punching or throwing nades goes all the way up to dropping .407 rebels which is a pretty solid improvement. Conversely, the Rebel Trooper gains literally nothing by being in melee and Grenades don't natively give them better dice, so their base number doesn't change.
For me, this basically means, Rebel Troopers generally shouldn't be equipped with Grenades at this point as those Grenades will only ever be worth it if it's ability comes up (blast or Impact) which is hard to guarantee. On the other hand, if Stormtroopers attack even once with Grenades, even if the ability isn't relevant for the attack (infantry in the open) you still get value out of those 5pts just in the upgrade over your white dice. The only scenario where Stormtroopers don't get any value out of Grenades at range 1 is where there is something with Armor and they Brought Concussion Grenades because upgrading the dice doesn't change the number of critical faces.
-The grunts best friend, Crew Served Weapons-
Z-6: 6 white dice with no surges
Average hits: 1.5
Average Crits: .75
Average Crits v Armor: .75
This weapon is the embodiment of "spray and pray". The Z-6 excels in 3 areas and they're all relevant. The first is the Crit rate against unarmored targets. This actually makes the Z-6 a good choice against troops camping cover and dodge tokens as the higher quality weapons on this list are more likely to just score normal hits that get negated whereas roughly half of all Z-6 damage results are crits. The second area is cost. At 22pts and being in a 40pt squad, it's easily the cheapest way to get dice on the table and that has a lot of value unto itself. The last are that the Z-6 excels is burst damage or the ability to do a lot of damage at once. While it's very unlikely, the Z-6 by itself can kill anything in the game except for Vader, the T-47 and the AT-ST in a single volley. This means that an army with a lot of Z-6's almost always has a chance to catch up if it falls behind in attrition to a degree that few other weapons can really match. Operation "Roll High" is a go!
MPL-57: 2 red dice with no surges
Average hits: 1.5
Average Crits: .25
Average Crits v Armor: 1.25
The major selling point the MPL has over the Z-6 is the .50 increase in Crits vs armor and the Ion effect. The average hits is the same across both weapons and the average crits v non-armored targets is much lower. It also has 2 other drawbacks over the Z-6 in the increased cost of 32pts and having to be exhausted when used. To put it simply, if you're not getting value out of the Ion tokens, the MPL just isn't worth it. In terms of raw damage vs Armor, it's not miles ahead of the Z-6 and while it's average damage v infantry is the same, it has nowhere near the burst damage. Any weapon that exhausts on use also has a number of other issues, it doesn't generally work while suppressed as when you only have 1 action, spending on recover is pretty rough even if it does also clear your suppression, and it's uncommon that you'll get more than 1 chance to fire an aimed shot with the weapon. Right now vehicles are going to be everywhere by simply virtue that they are the only non-trooper, non-commander options available, but if we see a serious decline in vehicles, the MPL-57 probably disappears along with them fairly rapidly.
DLT-19: 2 red dice with surges to hits
Average hits: 1.75
Average Crits: .25
Average Crits v Armor: 1.25
This is "Old reliable". The combination of Range 4, which neither Rebel Trooper heavy weapons posses, Red dice with a surge for high accuracy and Impact 1 to allow it to threaten most targets, the DLT is there to establish dominance and maintain it for the rest of the game and with the moderate cost of 24pts, it's an absolute steal. It only 1 issue and that's that it's high reliability can actually backfire. If the attrition takes a turn for the worst, the DLT being capped at 2 damage won't allow you to catch back up with this weapon. If you fall behind, you're going to stay behind. This isn't that severe, but you do have to realize it and have a plan for something else to make up the difference if it turns south. It's not a serious drawback and it doesn't detract from this generally amazing weapon at all.
HH-12: 3 black dice with surges to hits
Average hits: 1.875
Average Crits: .375
Average Crits v Armor: 1.875
So, this is the best damage numbers we've seen so far outside of the average crits v non armor. It's also notable that when the HH-12 is in a squad the Impact of 3 will frequently mean 3-4 crits making it easily the most damaging weapon against vehicles by far. All of that does come with some daunting costs. It has the same problem of the MPL in terms of exhausting which by itself is not great. It also has the Cumbersome rule so it can't move and shoot. this is noteworthy because it decreases the range i.e. a Z-6 unit only has range 3, but can add a walk on top of that or a unit of Stormtroopers with the DLT can take 2 walks and shoot at range 4 with the "New Ways to Motivate Them" command card. This makes it's range more hard and fast than with every other weapon on this list. It also costs 34pts making it the most expensive heavy weapon in the game at this moment. The HH-12 is a really tempting weapon simply due to the raw math, but it's drawbacks are also quite substantial. It's at its best against AT-RTs that can easily be crippled by a single volley from a squad equipped with the HH-12, but it typically loses damage against the T-47 due to cover negating hits that could be made crits with Impact. An AT-ST will also be regretting its poor life choices if it gets hit by 1 or more of these but won't be seriously threatened by 1-2 shots alone. The drawbacks will probably be enough to make these weapons also disappear relatively quickly if Armor becomes a less common keyword, but the upside of this weapon when confronted with Armor is so high that if Armor spam becomes a thing, we might very see these pop up everywhere.
-Some fire support, AT-RT and Speeder Bike weapons-
For the sake of brevity (ha, brevity), we're skipping the A-300 and holdout blasters. The TL:DR for those weapons is that the A-300 a marginally better E-11 that rolls 2 dice and the holdout blaster is a 2 shot A-280 with less range and surges to hits. It is noteworthy that the AT-RT guns, outside of the A-300 and melee attack are all fixed front making setup very important if you don't want to constantly be spending actions just to get targets in your arc.
Flamethrower: 2 black dice per model with surges to crits
Average hits: 1.25 per model
Average Crits: .5 per model
Average Crits v Armor: .5 per model
So this weapon is highly variable. Against single model units, it's underwhelming to the point of not really being worth it. Against larger units with 1wound per model, it's pretty devastating. It has a 62.5% chance of killing each Stormtrooper in a unit and an 82.5% chance of killing each rebel trooper in a unit (factoring in defense dice on both). This, combined with cover denial, makes the flamethrower a tempting weapon, but it's so ineffectual against anything that has a small model count and multiple wounds (literally everything but troopers right now) and has such close range that we probably won't see a whole lot of them...on the other hand, some people do just want to watch the world burn and troopers are very important for objectives.
Laser Cannon: 2 black and 1 red with surges to crits
Average hits: 2.075
Average Crits: .75
Average Crits v Armor: 2.075
The Laser Cannon is definitely the Armor hunting weapon with Impact 3, but can also be effective against infantry just because the AT-RT surges to crits, so even infantry spamming dodge and cover will often find themselves getting tagged by the Laser Cannon's silver laser bolt. It's also the only weapon on the AT-RT that reaches out to range 4 even if it can't shoot at targets at range 1. It also doesn't really have the burst potential you'll be looking for on a 90pt model. In that way, it reminds me of the DLT, a reliable weapon that's pretty good at most things and has good range, but won't help you overcome a deficit in attrition if one arises.
Rotary Cannon: 5 black dice with surges to crits
Average hits: 3.125
Average Crits: 1.25
Average Crits v Armor: 1.25
This is probably going to be the go-to weapon for most players. It's useful in basically any role because of the combination of 5 dice and the fact that it surges to crits allowing it to pinch hit as an Armor hunter as needed. It doesn't quite have the range of the Laser Cannon which forces the Rotary Cannon to play somewhat further up and lacking the Impact 3, the damage against Armor is not as guaranteed, it also doesn't have the raw infantry murder of the flamethrower, but it's never rendered completely useless.
Grappling Claws: 3 red dice with surges to crits
Average hits: 2.475
Average Crits: .75
Average Crits v Armor: 1.75
...what the actual ****? Why is the AT-RT's curb stomp attack so good? This is better than all of the infantry heavy weapons and it's just the AT-RT kicking you. The only melee attack that beats this melee attack is lightsabers. Why does it have Impact 1 as well? If it wasn't for the fact that vehicle movement is not the best for trying get into melee and that vehicles can still be shot in melee, I'd totally spam 3 naked AT-RTs and just curb stomp everything to death. My soul hurts.
Ax-20 Blaster Cannon: 1 red, 1 black and 1 white dice with surges to hits
Average hits: 1.875
Average Crits: .375
Average Crits v Armor: 1.375
Overall a good weapon. It's has the dice per model to give some good burst potential, has a good average against Armor and is helped by the fact that it's very easy for the Speeder Bikes to line up the shots with their compulsory move and use their normal actions to just aim and fire. The only meaningful drawback is the fixed front rule, but this is easily mitigated by good planning.
-Roll out the big guns, T-47 and AT-ST weapons-
It's noteworthy that every AT-ST weapon is locked in the front making the AT-ST similar to the AT-RT in terms of how important it is to properly line up your arcs to avoid having to do excessive turning around. It should also be noted that the AT-ST can stack dice and keywords relatively effectively assuming it has the weapons to do so.
Ap/11 Double Laser Cannon: 3 red and 3 black dice with no surges
Average hits: 3.75
Average Crits: .75
Average Crits v Armor: 3.75
So these are the best numbers of anything on the list so far with the exception of the Rotary Blaster beating it out on natural crits. That's to be expected, just buying a T-47 with this gun runs you 175 without any upgrades. Much like the speeder bikes, the only noteworthy drawback is the fixed front rule and, as with them, this is easily mitigated by good planning. A good weapon with great averages even on naked dice and utility against basically any target, but the platform is fairly expensive, so this is to be expected.
Mo/DK Power Harpoon: 1 red die with no surges
Average hits: .75
Average Crits: .125
Average Crits v Armor: .75
So, it's an 8pt weapon. There's not really much else to say about the Harpoon. With a single die, it does very little damage. The Tow Cable ability is at its best against AT-STs and AT-RTs because neither of those vehicles can adjust their facings with a compulsory move like the T-47 or Speeder Bikes (and also Speeder Bikes have cover, so unless you hard roll the crit it can't even hurt them through cover) and will have to spend an action. You can't go that wrong for an 8pt gun that can potentially throw suppression out, but your plan should not involve this weapon, it should be a happy bonus.
Ground Buzzer: 4 black dice with no surges
Average hits: 2
Average Crits: .5
Average Crits v Armor: .5
It has decent burst damage with a potential to get up to 4 hits and an average of 2 hits isn't bad at all. It's not the best against Armor or troops in cover as you only really hurt them through natural crits. It's not bad, but it's 20pts with a limited range and it's fixed rear while your primary is fixed front. meaning that using both of them at once involves your T-47 being surrounded which is usually anti-good.
MS-4 Twin Blaster Cannon: 2 red, 2 black and 2 white with no surges
Average hits: 3
Average Crits: .75
Average Crits v Armor: 3
This weapon's real strength is that its range goes out to 4. It's more expensive than the T-47 and worse in every area with the exception of natural crits where they're tied. With those white dice in the pool, is a fair bit less good both on average and because if you aim, you're frequently re-rolling whites into more blanks. It's a good weapon to be sure, but a lot of the AT-STs points are for the 11 health of Armor and not necessarily this weapon.
DW-3 Concussion Grenade Launcher: 2 black dice with no surges
Average hits: 1
Average Crits: .25
Average Crits v Armor: .25
The real reason you take this weapon is to ignore cover with your attacks as that can effectively add 1-2 to your damage against dug in troops alone. The range of 2 is not great especially because the AT-ST is not usually a fan of doing a lot of moving, but at 15pts it's not a bank breaker. In the right situation, solid, but doesn't play to the base of fire role of the AT-ST as well as other upgrades.
88 Twin Light Blaster Cannon: 1 red, 1 black and 1 white dice with no surges
Average hits: 1.5
Average Crits: .375
Average Crits v Armor: 1.375
This is basically a sawed off main gun. It has a little less range at only 3, but has the same dice distribution and will add another point of impact. Pretty solid generalist weapon and hard to go truly wrong with it.
Mortar Launcher: 3 white dice with no surges
Average hits: .75
Average Crits: .375
Average Crits v Armor: .375
So, by far the worst damage and can't fire at targets that are within range 3. It's essentially just there to throw out suppression as it will struggle to consistently kill even basic troops in the open and almost worthless against Armor as those targets don't care about the suppression. It is cheap and suppression spam can be effective especially if you catch a unit away from their commander, but don't expect much from it.
Fence Cutting Blades: 4 red dice with no surges
Average hits: 3
Average Crits: .5
Average Crits v Armor: .5
It's no AT-RT, but this attack will still hurt.
The top performer in each category:
Average hits: 3.75 (T-47 primary weapon)*
Average Crits: 1.25 (Rotary Cannon)*
Average Crits v Armor: 3.75 (T-47 primary weapon)
There is an Asterisk in that the AT-RT could win categories 1 and 2 with the Flamethrower against a sufficiently large unit, but it's not guaranteed and it's also pretty noteworthy that the AT-ST's ability to modify it's attack pool with key abilities or dice as needed gives it more firepower the raw numbers show (at the cost of more points for the extra weapons and possibly Weiss). It's also noteworthy that the heavy weapons will almost always be augmented by squad dice, but this is mostly only relevant for the HH-12 as it's the only one with an Impact ability high enough to make a big difference.