PvP or PvE?
My experiences so far are mainly PvE, but Blood DKs, Demon Hunters are both really strong from a soloing standpoint. If you don't have people to do PvE with, something with sustain is important for soloing bosses at a decent pace without waiting for a bunch of cooldowns. Hunters seem pretty solid as well, and I'm enjoying my enhancement shaman personally, but I haven't gotten to match up against a lot of people yet. Outlaw rogues are hilarious in world PvP, critting people for well over a million with whatever that one gun combo skill is.
That being said, there is a point of terminal velocity where you will roll over the content like crazy. As an Enhancement shaman, I was able to solo neltharions lair pretty easily starting at around ilvl 760. Your mileage my vary.