This is a tough one. I think I'll just settle for the first five I can think of:
Blazing Sword: Hector, Lyndis, Guy, Kent, Ninian (Sain would've made it here too, but I think Kent's skill is better, even though they're pretty similar.)
Sacred Stones: Eirika, Ephraim, Innes, Seth, Joshua (Eirika and Ephraim on a horse might've been better Seth. I'm still not entirely sure, though.)
Path of Radiance: Ike, Soren, Lethe, Mia, Janulf (I think I would've chosen Tibarn over Janulf, but since he doesn't really get a lot of usage here....)
Shadow Dragon: Marth, Caeda, Merric, Ogma, Bord (Only because Bord seemed to get more kills than Cord and Barst. Those guys could be triplets!)
As for overall unit ability, I love calvary/champion units. They're pretty unstoppable, unless you get stuck on a board with bad weather/terrain. You also don't have to worry about getting picked off by archers like with the flying units.
In Path of Radiance, I think the cat laguz were pretty darn similar in skill. I always found a place somewhere for Lethe and Ranulf in my army.