Personal preference aside, fun in wow pvp tends to be balanced between survivability/damage capability and variety.
In terms of what they can do, the classes that people tend to stick with longest are as pre mentioned are Rogue, Mage, Preist... across 2/3 specs they always seem to be viable if not very strong and 'fun' to play.
Those three classes tick all of those boxes with slight hang ups and always start and finish very strong each patch/season/expansion. When you go over them with a fine-tooth comb rogues have suffered from lots of bugs in the past and still now, preist have suffered from lock outs, mage suffering from dispels but numbers show they've been the few classes have stuck with for a long time and still enjoyed themselves with.
And once in a while you'll try a different class and for whatever reason you seem to enjoy it even if its not the best or does the most damage, it could be a certain ability, it could just be what you can do with them outside of pvp as well or anything so the best advice i'd give is to try all the classes out. At this point i've got one of everything if not twice over and once in a while I'll go through a phase and enjoy one toon more than the other and of course play that one basically all the time.
Holy Pally, MM/BM Hunter, Feral Druid, Disco Preist, i've had fun on and been flavor of the month for me recently, i.e. now im really feeling my frost dk in pvp, its a spec I've wanted to be good in pvp for some time now (which it is atm) because largely its the only spec of a dks im really interested by but sooner or later I'll get bored of him too probably re-roll again.
On a level, the most fun in pvp is pvp itself, roll with the times, roll and re-roll them all and you'll probably find that one long term class you enjoy or at the very least keep pvping, learning other classes and end up like me who's still having a blast in pvp no matter what toon i'm on but not really having a main atm.
<-- warrior was my main in wotlk, i dont like how they are now so he's been left since then and I've not really found a new main again since then but I'm still having fun on whatever I play and thats the main thing.