Are you looking just based on performance? Then it’s DH on your list. Granted that answer can change quickly if they ever throw out any balance changes.
Just want to play the most broken thing currently? DH.
Want to play an insanely strong spec all around that is a bit too strong atm? Shadow priest.
Want to play a class that has multiple insanely strong specs and is relatively simple? Warlock.
Want to play SL season1? Sub rogue.
Want to play mage and be blizzard’s favored child? Arcane mage. (frost is ok too)
Want to walk around the arena and pretend pressing death strike is skillful counter play and be damn near unkillable? Unholy DK.