I stopped playing wow about 5 months ago, now i want to start playing again and i was wondering wich was the best spec for PVP and also how is currently the warlock in PVE, specialy demonology. Thanks guys
Affliction is the only viable spec in arena. Unfortunately it's purely a support class this expansion, with poor single target damage and poor survivability. The only reason they're viable in arena is the dispel protection and convenience of a portal making them hard to attack. And of course being a fear bot. Nobody fears their damage, though, so if you want to feel like you're actually killing someone, roll something else.
As for battlegrounds, any spec is viable in the hands of a good player, but you'll find warlocks are weaker all around here than in arena. Unlike arena, maps aren't designed for portal abuse, so people will actually go after you, and with multiple healers nobody's afraid to dispel either. It's unfortunate that people are so brainwashed with the idea of arena representation determining class balance that they can't see what's actually overpowered and what's not.
Can't comment on raiding though, sorry. That's not an area of the game I dabble in.