Updated for patch: 5.3
Major Payne's Pets Are:
Grizzle (Beast) Level 25 Elite
Bash, Hibernate, Rampage
As for 5.3, Grizzle is usually the first opponent and typically starts out with a 3 round Ramage, his Hibernate ability is so strong you might the best mileage using a Mechanical pet instead of a Flying one. Since the recent changes have nullified the Fluxfire Feline I've started using the Clockwork Gnome, his combination of Metal Fist and Build Turret will allow him quickly dispatch Grizzle and take a good bite out of the next opponent too.
Beakmaster X-225 (Mechanical) Level 25 Elite
Batter, Shock and Awe, Wind-Up
I've found that the Water Waveling works well against the Beakmaster, but most Elemental types will work here as they output more damage and take less.(Possibly subject to change in the 5.4 patch)
Bloom (Elemental) Level 25 Rare
Entangling Roots, Lash, Soothing Mists
A snail type battle pet such as Silkbead Snail is one of the better opponents for Bloom due to the decrease damage and the Dive ability being both Aquatic damage (bonus against Elemental pets) in addition to a good way to avoid the significant damage from Entangling Roots.