If you suck at playing your spec, it doesn't matter.
If you play a spec that sucks and isn't fun, it doesn't matter.
If both, why PvP?
Pretty much all specs are strong in PvP. Historically, maybe only Druids and Shamans have been "bad," with Elemental and Balance having no defensive or escape capability for the longest time. Even then, mastering them in PvP just made you that much better than everyone else when playing "easy" specs. What you're asking for is which class has the highest success rate. I don't know of a statistic, but I'm going to go with Mages, since they just have everything.
Priests are usually fairly stable (Either Disc or Shadow - one of them always seems to be doing well). Mages haven't had any real dire patches so far either that I can recall.
Stay away from the big 3 hybrids: Paladins, Shamans and Druids. If your spec gets nerfed or falls behind (which happens fairly often every expac cycle guaranteed) you have to totally re-gear and learn a new style/mindset.
Mages are usually strong, but they're actually fairly hard to play despite all the 1500 rated players saying how easy it is.
Priests are good almost every season, if Shadow is bad Disc is usually strong and vice versa, been like that since BC really.
Warlocks seem to be good all the time as well, not really broken, but multi dotting is so strong that Affliction will almost always be good paired with a Mage/DK.
Rogues have been pretty good as well, they were mediocre in 5.0/5.1 but I think that was more other classes were too strong. The amount of control a Rogue brings is absurd, but they're also pretty difficult to play well.
Those four seem to be the classes that are always viable.
the least suckage? just my opinion on the subject but id have to say that from my experience rogues, warriors, locks, priests and mages have been fairly stable. again, just from my experience and im by no means an expert or pro. i was able to maintain a 2-2.1k rating in 2's and 3's in s1-3 and then basically stopped taking arenas serious once s4 started and never really got back into it as most of my pvp friends stopped playing.
Rogue Mage Priest are pretty much the most stable in PVP and embody the three PVP roles best: melee dps, magic/range dps, healer. RMP as a comp has been one of the strongest in arena throughout as well, with good reason.
All three of these classes rarely have bad seasons as at least one spec of theirs is excellent for pvp.
i was considering mage/rogue, ive played both. tho i prefer melee, and right now rogues may be in a decent place, but they do such poor damage outside of CD's it seems
But i think those 2 are going to be my leanings, im looking at monks as well, tho ive heard very different bits of info on whether they were good or not. how are these classes in say...BG's, i know arenas are "what matters" but for times when you just want to have fun and do some bg's with a friend...what would you pick? mage, rogue, or monk?