Well, in terms of damage,
Fluxfire Feline is probably one of the most strongest ones, as I have seen 2000+ non-crit from infamous Supercharge + Wind-Up combo. Just keep in mind that Blizzard hinted that there might be nerf coming to this pet. In theory
Legs is also pretty strong, as you can time Pump, Whirlpool and Surge of Power for pretty respectable burst.
In terms of control, I think that
Hopling is pretty good one. With over 300 speed at max level it can out-speed many other pets, Backflip allows you to interrupt (if each one hits and goes first) 33% of your opponents turns, Sticky Goo roots for
5 turns and Tongue Lash works well against those pesky Undeads.
Just my 2 coppers.