Pilot skills, to be precise.
We have a range of 0-12 in this game, through various pilot abilities and upgrades.

I will grant that between individual ships, a higher PS gives you the combat advantage, and gets to react to the maneuver of the lower PS opponent (at the risk of collision).

However, against an Academy Pilot, Vader may as well be a level 2. Those other 7 levels aren't doing anything for him there.

Now, Level 9 still has those advantages over Level 8, and so on and so forth. That is all well and good.
However, with a higher PS comes a higher price, and with a higher price comes a greater rarity.
You can fit 4 X-Wings in a fleet, but one of them gets lost if you go with the pilots of Level 7 and higher.

Where's the ideal place to stop, so you have an advantage over most of the ships you'll face, but not waste points in the match-up by over-bidding?

Depends on the ship. Pilot skill on a ship with little to no positioning ability barely matter beyond the turn your ship dies (in most cases). More specifically they become a disadvantage due to a weakness to action denial.

Meanwhile ships with positioning abilities and effects that trigger on attacking need high pilot skill. This includes all Phantoms using Advanced Cloaking device, Turr Phennir, and Cracken.

The fact is that you want the lowest pilot skill advantage you can get and most large base ships want low pilot skill.

Against Phantoms you want to move after them and shoot before them.

Basically. This has no answer, but Shuttles and Firesprays are better with lower Pilot skill, as are blocking ships like Tie fighters.

This is where Cracken shines for me. For 21 points he's buffing a friendly to 8. Cheapest rebel pilot for swarm tactics. Doing so you have two sips shooting before 85% of the other pilots out there. Plus I'm terrified of predator, or better yet I want to limit it's damage. So I've been focusing on max ships with responsible ps.

There's no 'perfect' answer. More mobile ships (Interceptors, A-Wings, Defenders, E-Wings, Phantoms come to mind) benefit having than others due to to knowing whether or not to barrel roll or boost, and better knowing which way to do it

For my high mobility ships I say 6+. For my minions 1-2 is fine. For the utility ships you want them shooting at 4. Also, having all your ships shoot before the popular PS4 just makes sense- eliminate a threat before it can respond = win (most of the time). Chewing up a basic shuttle or Firespray will also become pretty important in the near future me thinks.

For a moment, we will assume that no pilot has a unique card ability, as they can radically affect the decision.

In a tournament or otherwise competitive setting, Pilot Skill is, quite simply, a betting game. You place a bid and pay a little more to bet than your ship can fire before someone else's. But, for that increased cost, you gain no extra firepower or durability, just the POSSIBILITY of winning a positioning or firing advantage over your opponents. In addition, lower Pilot Skills are more common than higher pilot skills, meaning that for each point you pay, your return diminishes, which comes back around to the betting game. Often, the more money you pay, the less your return is in comparison, even if you win.

Then, there are the unique Pilot Abilities. Some abilities are quite good, but you pay a premium to use them. They come with a higher Pilot Skill than the generic pilots, and are either offensive, defensive, or supportive. The specifics of each ability make it "worth" more or less points to many players, but you often end up with a ship with the same durability for a higher price, so the buy-in is similar to the Pilot Skill bid, with a little higher return.

I often find that the extra 2 points for the midway generic are worth it at tournaments. Most ships used in the competitive scene are PS4 or lower, so ships that are PS4 or 5 can be worth those extra points for the firing advantage, barring certain exceptions. I really feel like Daggers or Reds are worth the two points over Blues or Rookies. However, the Bounty Hunter is pretty good value for its price, and it has 3 named pilots. So, it is really up to the player if those abilities are worth the price difference. I also feel like the PS1 Defender, now that I've seen it played, is fair value for its cost, but I don't agree that the PS3 Defender is worth it. Small difference, I know, but for one more point over the PS3, you can have a Bounty Hunter. A price difference of three points on a ship could decide if you can fit another ship or upgrade, but a one point difference can rarely offer that. Therefore, for one more point, I'll swap it for a Firespray, instead of saving for a fourth or fifth ship. I also think its pretty difficult to justify spending more than 12 points on a generic TIE Fighter. Some of the named TIE Fighter pilots have criminally underrated abilities, but the only one I see played much is Howlrunner. There is the occasional Mauler Mithel, but it's usually Howlrunner or Academies. My favorite TIE Fighter list to run is the six named TIE Swarm. They are all cheap and they have useful abilities, but again, if I'm not getting a named pilot, TIE Fighters aren't worth the extra points. I am still holding judgment on Z95s until I get to play them more, but I doubt I'll ever pay the extra point for a Tala. Honestly, I even think it's difficult to justify a missile on the generic Zs, as you are increasing the price of a weak, fragile ship for an unreliable single attack.

Just my thoughts.

Another thing to consider though is most high PS pilots have other abilities that make them worth using besides just the Raw PS so that will usually play into the cost vs PS analysis