You forgot RV.
I'd have to go with Sirens only cuz its the most populated. I like layout of bloody bay as well. Warburg is nice but can be a tad laggy on some machines. RV, to many boss npc's and I don't like heavies, I find they ruin the whole PvP feel.
Siren's Call. No question about it.
There's almost always someone in there to pummel, and if not, you have the hot spots to help out on.
@Moonshade - too many to list anymore
Siren's is my favorite. Bloody Bay just lacks the punch and is way too spread out. Warburg has no sides, which makes it usually too chaotic to be fun. And RV is still lacking people who play there.
Siren's is clear-cut and has people fighting without their high-end powers.
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Sirens Call used to be my favourite but now its definetly Statesman's Victory. All those Boss NPC's really make you feel that "you've made it into the big leagues". I just cant wait until more villains get into the zone. Great zone.
@SinghMC
I would say sirens call is my favourite because i feel in the higher lvl pvp zones fights depend (especially on brutes and scrappers) on the lvl 38 powers from your second set, i hate fights that revolve around whoever uses there lvl 38 power first loses 
SIrens is the only zone where people are actually encouraged to enter pvp combat. In other zones they dont gain anything (apart from badges) from it. Hence it is the most populated. The devs might give this a bit of thought.
arena all the way, the zones are gr8 for practising and learning but the arena is where the real PVP is at
Bloody Bay, because it's got good atmosphere and it's friendlier for solo and more casual PvPers. Warburg comes in at second place, Siren's at last place. I do like Siren's a lot, but I've lost count of how many times I've been killed for merely popping my head above the parapet.
I haven't played the Arena enough to make a judgement, and I'm one of the few players who can't enter Recluse's Victory yet. I did play the RV test event though, and it was a lot of fun.
Well, if i feel tired and want to go pwn some no0bs. Sirens call. if i feel like owning some badge hunters...hehe RV, and if i feel like some hardcore pvp, arena 
but cant forget Warburg, got to be my fav pvp zone!
Warburg, crap map but better level range and dynamics
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Arena. Always Arena.
I think all the zones are "broken" to some degree in that the game design works very poorly for most zones, including at the moment Recluse's Victory.
I've noticed recently, on Union at least, some zones degenerating into a "1v1" duelling area which is the nail in the coffin. There just isn't a compelling enough mechanism to get players involved in what is going on in the zone, in fact as far as Siren's goes, the opposite.
That said Siren's pretty much works out as the "best of the worst" for me as the level is reasonable and there are tangible rewards for fighting. It's a shame Warburg is so mixed as I prefer Heroes Vs Villains, it's also even worse for the "1v1" duelling nonsense.
Recluse's victory seems like it has some potential to be fun but has an "all or nothing" reward scheme, no missions to do, no temp powers except for the mini-bot which doesn't make it especially attractive. I can't see it being free of the Badgers any time soon aside from player organised events to fight in the zone.
All of which seems a shame to me since I've found PvP in general a lot of fun and much more challenging than PvE. I think Cryptic have in fact done a reasonable job of shoe-horning PvP into the game but personally I would prefer some real PvP mini-games to play inside the zone other than the free form fights that we have now which are fun enough but pretty shallow.
By "mini-games" I mean - king of the hill, capture the flag, territories, that kind of stuff. Difficult to make work in a drop-in PvP zone? maybe, but I think Crytpic could do better.
Edit: I'm being a bit thick, RV is what I would call 'territories' 
, so has the most potential from my point of view. Some more balancing mechanisms might be nice, like maybe Elite Boss level villain/hero NPCs *defending* a pillbox as well as the big guys attacking. Something like the Civic Squad zapping in to defend a pillbox. High level villain population or interest isn't there yet though.
Whats Your favourite PvP Area ?
Bloody Bay?
Sirens Call?
Warburg?
OR the arena