"damage" -> rule out Shaman and Druid
"consistent" -> rule out Shadow Priest
"don't want to depend on others in pvp scenarios" -> rule out Hunter (in current state Hunter sucks 1v1 because FD bugs)
So that leaves you with Warlock and Mage. Do some research on what they're like. PvP: Mages are super mobile, have tons of control and are easy to feel good at. Warlocks are super immobile, are very tanky and nukey and have a pretty steep learning curve (it's going to take a lot of experience before you start to feel like you rock).
PvE: Mages have a better single target rotation, because it at least changes from Frostbolt spam to Ignite management depending on content. Warlocks just spam Shadowbolt and Lifetap for the entire game, and perhaps Corruption if you get alloted a debuff slot.