PvP i love my cat. I'm still using Humar to this day (level 66 hunter), never got tired of him... http://petopia.brashendeavors.net/html/mobs/mob_humarthepridelord.shtml
He has Bite 9, Claw 9, Growl and Dash 3. Can close the distance very quickly with Dash, can pop frenzy very fast, and hits pretty hard too. Growl I keep on during PvE.
I don't think a Spider would be a bad pet either, it just depends on your style. I'd prefer a scorpion myself, I think they look cool, but a spider works too. I think spiders are somewhat lacking in that they can only learn Bite, Cower Growl. It Would be nice if they had a poison attack too. So as far as offensive pets go, I think Spiders are somewhat lacking. Cats, Windserpents, Bats, Birds, and Raptors are a bit more versatile as they can learn more skills. I wouldn't use a Spider for PvP though, the lack of a quick intercept attack (dash, charge, dive) really hurts.
A pet I think is a bit rarer and looks great is the deviate serpent, which has poison spit, a very useful pvp skill especially against Rogues...