Demo lock, hands down, it's not a "pop whatever's off CD to win" type of class - you actually need to know the fight, take the overall raid DPS into account and think about when to use Metamorphosis, Immo Aura etc. It can change every week, depending on your raid group setup.
Warlocks also have the most badass utility spells - Soulstone and Healthstones can make a huge difference between a wipe and kill, Dark Pact is the best SP buff in the game and Ritual of Summoning saves so much time and frustration, especially on molten xD
For melee I'd say ret paladin, but not the boring "spam all the things, lol 30K DPS" type of playstyle... Get an addon like HealBot or w/e, bind every utility spell (all the hands, heals and sнit) and use them!
98,7% of rets on this server don't do anything besides pure DPS, they aren't supporting the raid in any way, it's so fuсking sad >.<
But if you just can't be bothered - roll a Shadow Priest, it literally sнits allover the other DPS classes saying "вicth, I don't care", with all the DMG reduction, self healing from VE, Dispersion, fade, you won't die unless it's a 100% wipe.
SP is the most laid back DPS spec IMO, no burst DPS cooldowns, no procs, just 2 DoTs and 1 filler - pure KEK. Plus there's noting better than tanking in Dispersion xD