Prior to 5.2, the Fluxfire Feline was easily the most overpowered pet. Since it was nerfed, it really varies. The Flayer Youngling is pretty great, even though Reflection was changed to Deflection; he still totally eats up critters, and critters are everywhere. I've recently leveled up a Scourged Whelpling (more common since 5.2, I've found, and an ideal counter to Dragonkin) and an Infinite Whelpling (hits like a truck), and an Emerald Proto-Whelp (perfect counter versus flying, PP breed puts some muscle into it, and it can have two heals). Mostly I just use perfect counters (ie, pets with abilities that do 50% more damage against a family, and take 33% less). I've mentioned them before, but...
-Anti-Critter: Flayer Youngling (Rampage tears critters apart; Reflection is just gravy on top)
-Anti-Mechanical: Amethyst Shale Hatchling (Any Hatchling from Deepholm will work; they have Stone Rush though, which hurts them and can make capture a pain. And it was slightly nerfed recently; 1 round cooldown now).
-Anti-Aquatic: Jade Owl (Feels a little weak, but if Lift-Off hits your enemy will be more than half dead. Have to use a stone to make it rare quality).
-Anti-Elemental: Rapana Whelk (Apparently a Shimmerscale Snail is better, but the Rapana Whelk is level 22-23 when you catch it. These things are BEASTS against elementals. This was my go-to choice for the Pandaren Spirit Trainers, and it could kill them easily... if Dive hit).
-Anti-flying: Celestial Dragon (But a lot of Dragon Whelps work here. Just need a good magic ability)
-Anti-humanoid: Twilight Spiderling (Any spider works; I prefer one with Brittle Webbing to Sticky Webbing because it does more damage, but Leech Life + Webbing is MONEY. Good self-healing and good damage).
-Anti-magic: Mechanical Pandaren Dragonling (Very interesting pet, with a lot of diverse abilities. Have to stone it to get a rare).
-Anti-Undead: Softshell Snapling (Almost any turtle works, but this is also kind of a weak counter: Powerball is a pretty weak move even doing 50% more damage).
The only one I don't have a perfect counter for is anti-beast, because as far as I know there's only one flying pet with a mechanical attack, and it's a TCG redeemed kite. Not shelling out RL money for tens of thousands of gold for that, when any mech in the game should be able to outlast a beast. I use a Clockwork Gnome for Major Payne's bear, for example.
Edit: I'm also not a fan of the anti-undead hard counter. Not only is Powerball weak, there are no power-oriented turtles. That means that fights are closer than they should be. This is slightly countered by the fact that the turtles will attack faster (after using Powerball), and have good damage mitigation or heals, but it still isn't fun. High power = bigger hits AND bigger heals. Edited, May 7th 2013 4:51pm by LockeColeMA
Retired News Writer for the ZAM Network
WoW - Aureliano the Insane - level 90 Druid on Sen'Jin
Nanaoki - level 90 Mage on Sen'Jin