Mechanical Pandaren Dragonling: Most useful pet for Draenor trainers. Get it from the AH and upgrade it with a stone.
Anubisath Guardian: AQ40 or AH. This thing absolutely eats anything that casts DoTs alive, as well as having a Deflect to avoid big hits.
Emerald Proto-Whelp: Tame it in Sholozar, preferably a P/P breed (although mine isn't and it has worked just fine). It has two heals, a flying move and the primary attack is strong against flying or magic pets. Teams up very well with Anubisath for most Pandaria trainers.
Chrominius: Has a very, very big hit as well as debuffing enemies to take extra damage. Hugely useful for pets with the Boss damage reduction. Drops in BWL/buy from AH.
Darkmoon Tonk: Similar in function to Chrominius. Darkmoon/AH.
Darkmoon Zeppelin: Similar to the Mechanical Pandaren Dragonling. Darkmoon/AH.
Ghostly Skull: A poor man's Val'kyr, can apply the very important Unholy Ascension. Buy in Dalaran.
Pandaren Water Spirit: This will probably be a bit more expensive to AH (or you can get it from the bags from Pandaria spirit trainers), but teams very well with Chrominius & Ghostly Skull to line up several huge hits to burst down annoying pets.

Anubisath & Emerald Proto-Whelp can effectively 2 man a huge amount of Pandaria pet trainers, making levelling other pets way quicker.