I've beaten all but Pandaria's grand master pet tamers with the same 3 pets (Celestial Dragon, Pterrordax Hatchling and Gregarious Grell).
Now as I'm leveling my pets I've noticed that Stunted Sharhorn, Clockwork Gnome, Darkmoon Monkey, Blighthawk, Purple Puffer, Dark Whelpling, Darkmoon Tonk, Darkmoon Turtls, Darkmoon Zeppelin, Emerald Whelpling, Lil'XT or Landro's Lil'XT, Lil' Deathwing, Nether Ray Fry, Pandaren Monk, Crimson Geode, Curious Wolvar Pup are all verry good pets.
I'm working on some good combo pets and will post them as I get it together. But here a short list of pets that work as combo team.
Any combo of these make a good team: Fire Beetle, Crimson Whelpling or Fel Flame & Lava Crab, Searing Scorchling, Lil' Ragnaros.
These two work well together: Terrible turnip, Ruby Sapling. Still trying to figure out more.