So, what are some of your favorite combos?

Here's a couple of mine:

Sudden Death - Altansar Rangers+Ambush Platform+Banshee Power Sword (Instant death to Bloodied non-SM Warlords.)

WAAAGH! Da Snotz - Snotling Attack+Battle Cry (Four 3/1 Snotz for picking off things.)

Here's a couple of mine:

Sudden Death - Altansar Rangers+Ambush Platform+Banshee Power Sword (Instant death to Bloodied non-SM Warlords.)

Banshee Power Sword can only be used against non warlord units; Ion Rifle would work though.

Cato + Talassarian Tempest Blade + one or more Catachan Outposts is nasty. 5+ unshieldable damage is a game-changer. Of course, to use it well, you need to either put him on the front lines or corral the enemy warlord, so it has its own issues...but man, is it satisfying to pull off!

I lost a game against a friend borrowing my Space Marine deck when I let Nazdreg get bloodied at the 6th planet instead of retreating him. My friend had initiative at the 7th planet, and I had nothing to take Cato out of the fight the next turn. Serious miscalculation on my part, but at least now I know what it's like to face that combo down from the other end.

Infernal Gateway & Umbral Preacher for when you guess correctly / corner the opponent Warlord and his "retinue" of Army Units, while having the damage advantage.

Edit:

And even though it is obvious, Promise of Dark Glory and Flesh Hounds are just sweet together

Umbral Preacher does not prevent the warlord's retreat, only the army units. But this does work very well under the right circumstances. Putting him in play with Infernal Gateway to surprise your opponent is nice too.

Stingwing Swarm + Ion Rifle (with or without Ambush Platform). HEADSHOT.

Possessed + Initiative token

Space Marine Dreadnought (or any other big hitter) + Plannum. Swing for the win, move to another battle, and repeat. Good times.

Umbral Preacher does not prevent the warlord's retreat, only the army units. But this does work very well under the right circumstances. Putting him in play with Infernal Gateway to surprise your opponent is nice too.

That is what I said, and crushing his "retinue" units that would normally arrive with him/her - leaves the Warlord without the normal units used for protection.