Ultimate Counterpets

[Aquilus]

I almost forgot my favorite aquatic with critter abilities is Leopard Tree Frog breed ID 5. Moveset (Tongue lash, Healing Wave, Frog Kiss) for pure undead countering I guess you would go with swarm of flies, but in general frog kiss is really nice.

This pet is the fastest frog in the game 325 at lvl 25 as rare quality. In addition to dealing lots of damage to undeads and taking reduced damage from them, his frog kiss and healing wave combo is good against a variety of pets, particularly tank like pets or pets that like to have long drawn out fights. Every time you frog kiss you have a 25% chance to make the pet lose 2 actions (if you attack first) and it also increases in damage with every attack. Healing Wave is just there to give you longer battles so that your frog kiss can get up to some epic damage levels. Not to mention the periodic polymorphs will definitely drag the battle out and give you 2 free turns basically. Try this guy out against a Mr. Pinchy 1 on 1 and you will start to see his role in your pet battles.

Note: The enemy team swapping pets resets the damage from Frog Kiss (not sure if this is intended or a bug, but does not seem consistent with the description.

Obviously avoid flyers like the plague.

Unfortunately frogs are very slow pets and even when you take flyers out of the picture there are still 110 pet/stat combinations that are faster than this little guy (at rare quality lvl 25). (that may sound bad but there are 836 combinations that are slower than him and if your opponent is the type that favors power then it is less likely that they will have a faster pet)

[Darkshroud]

Another thing that makes your undead counter even better Is that frog kiss will prevent the undead from reviving for one last free hit. Tho I'm pretty sure that's a bug...

[Aquilus]

I just looked and the Leopard Tree Frog is faster than every undead in the game except for Mr. Bigglesworth (same speed of 325).

Incidentally, the next fastest undead in the game are Voodoo Figurine, Giant Bone Spider & Fungal Abomination at 289 which is tied with the next fastest frogs (Mojo, Small Frog and Wood Frog (breed 11 or 21). Of those 3 frogs 2 have the same power as Leopard Tree Frog and more HP. Small frog has 13 more Power than leopard tree frog. So unless you're a huge fan of high HP pets I think Leopard Tree Frog is definitely the way to go.

Also, that's pretty sweet about the frog form negating undead racial. Very unpredictable but just icing on the cake.

[Phoenixap]

While not readily available... I love countering beasts with my Rocket Chicken.

[Poofah]

Baby Ape used to be my favorite anti-beast, but they stealth changed Barrel Toss to a beast attack rather than mechanical. I found out the hard way on the earth elemental tamer.

Emerald Proto-Whelp is the new anti-flyer in my opinion. He can have the same moveset that makes Emerald Whelpling so good vs flyers, but with 81 extra power at the cost of speed and some health. He can solo the air elemental tamer without breaking a sweat -- to be fair, emerald whelpling can too, but the proto is just more brutal.

[Enzzo]

These are pretty much the same conclusions I came to. To expand on this, look at the hardest tamers and see what kind of pets they have, and how to counter them.

For example, against Whispering Pandaren Spirit's pet Dusty, the critter pet with flying abilities, you will want a dragonkin with beast attacks, like an Azure Whelpling or a Dragonhawk.