Blood DK / 3x BM hunter / Healer of choice is a very strong option. Druid is strong but I couldn't get the heals to land to my liking, Shaman is a solid spammable healer, Monk is what I have been using in the past for the 5% melee damage bonus, but I am leaning towards trying out a Priest and seeing where that goes.

Resto Shaman / 4x BM hunter is extremely potent, but due to not having a true tank - you are limited on some fight mechanics that require one for positioning.

DH tank / 4x Boomkin - this is one I always try to get to work this expansion, and really want to but end up shelving it after a week or two and going back to the previous 2 options. 8.2 just buffed DH tanking so it may be back on the table again.
Biggest weakness is low burst AoE and mobility. Otherwise it also works as a PvP viable group as well.