the single best beast, despite what many people say about the situation dictating the response, is Xufu. his moves work with the beast passive better than any other pet in the game works with their passives. obviously he won't be a great choice against an NPC tamer with a strong mechanical; however in pvp he is absolutely monstrous. most go with prowl > feed, i used too, but now i realize the usefulness of moonlight as a guard against teams that rely on weather effects, crows, and making sure his spirit claws lands. he's on 3 of my 5 strongest pvp teams.
to avoid overlapping your already leveled pets i suggest "favorite"ing your current 25's. then as u figure out a pet to level, put it on your favorite list as well and level it from there. remember to research which breeds are the truly strong ones for each pet. such as the p/p DAH is pretty awful, while the S/S DAH is damn near godlike. the B/B val'kyr will beat the H/H val'kyr if they both haunt on the same round, etc...
pvp pet battles are very random. it's much like playing cards with nothing to bet, so you really have nothing to lose. you'll only gain a lot of knowledge and strategy that you probably wouldn't have otherwise figured out on your own, or even from here lol.
the first 2 pets you need for the tournament are pandaren water spirit and chrominius. they beat almost every BoF on their own and 3 of the 4 celestials (barring RNG hate.) i also suggest buying the alterac brew pup; he has the best backline pet heal for keeping you from quitting the tournament to heal your pets: when you seem to be losing, just run away and load the brew pup and fight one of the tamers (there's always one) that doesn't attack on the first move, then you use the brew pup's backline heal, run away...rinse repeat as needed.