Here are some nice, all-around PvP pets. Most of these are very easy to get, or are cheap on the AH. I will put the breeds that I prefer in () after the name. For brevity, I will only list 10. I won't list the FotM/OP stuff as there are enough of them saturating the queues as it is:
1. Emperor Crab(p/p) - Needed to help cancel out the FotM stuff. Really, about any crab will do but I think the P/P version of the emperor is the most useful. He's great against Undead (i.e. Valks), decent against the Death Adders (if running with Surge), and is very durable. He also has the added perk of being able to solo the occasional Sunlight Stall team that you will run into. My most satisfying wins are out-stalling the stall teams with one pet and watching them eventually flee
2. Blue Clockwork Rocket Bot(s/s) - This guy is for strategy lovers, and he packs a punch when used correctly.
3. Lil' Bling (s/s) - I have been battling for a while now, and I still find new uses for this guy. Not a prime dmg dealer, but very fast and durable, and another highly strategic pet.
4. Scourged Whelpling (h/h) - This guy can fill myriad gaps on myriad different teams. I currently love using him in a DoT-Regen team.
5. Gilnean Raven/Crow - I didn't list a breed because the flyer racial can overlook speed deficiencies for a few hits. This guy's speed and raw dmg capability make him a very common sight.
6. Spring Rabbit/Tolai Hare (s/s) - Faster that pretty much anything in the game by default other than above 50% health flyers and a cheetah that nobody uses. Great evasion moves and strong vs. undead as well.
7. Fiendish Imp(s/s) - Very usefull force-swapper and backling dmg dealer (with the bonus of backline heals from human racial!).
8. Moths - Racial being the same as other flyers, makes breeds a bit more flexible. I don't use them, but they can be highly effective against many things.
9. Nether Fairy Dragon (s/s) - Another guy that can fill many gaps on a team. He can be used as a setup for a Moonlight team, a Cyclone team, is pretty fast, and is a pretty good anti-flyer in his own right.
10. Fossilized Hatchling - He does very decent dmg against pretty much anything, has a heal, the Undead racial, and is a backline killer.
There are loads more, but some combination of the above should net positive results if played well. Of the above pets, my strongest team is likely the Emperor Crab/Fiendish Imp/Fossilized hatchling. This team is good against so many different things. For example, the crab is good against Undead and Elementals (when running with Surge which I almost always do), and can handle Sunlight and DoT teams no problem. The Imp is great against Mechs, and a good option against birds (Nether Gate being strong against flyers), and generally gets a decent amount of the health that he lost getting set up returned from his backline dmg. Finally, the Hatchling is great vs. Humanoids, and can decimate backline pets if he is not taken out quickly. So many combos out there. Half the fun is creating new teams to see what works well!