Warlock - very powerful right now on both 2s and 3s, have a lot of viable comps and two viable specs, scalles very well with gear. Many people believe that the most powerful comp this season will be LSD, so it is good bet. I played against some LSD 2.0 last season and boy, was that annoying (fear, fear, coil, bash, cyclone ><), pretty sure will still be viable this season.
Paladin - pretty weak right now, the only viable comp is with hunter/priest. The main problem is low survivability that won't scale with gear, the burst is quite strong.
Hunter - very strong (good damage and very good CC) and have quite a lot of viable comps (Thug cleave, jungle cleave, beast cleave and KFC).
Monk - very strong on 2s. Haven't been really strong on 3s on former season, but people says it scales very, very well at the current gear level. As far as know they play with hunter/healer (KFChi) and s. priest / shaman (there is a video on this comp by tosan on youtube that you can check).
Mage - IMO the strongest DPS on the game right now. It is ridiculous powerful with rogues on 2s. On 3s can play RMP, RMD (Easy and very powerful comp), FMP or God comp.
Shaman - Enhancement has the best set up damage on game right now, but survival isn't really good, still seems to be viable on 2s and 3s (Turbo cleave and Beast cleave were very strong last season, can't say much about now). Elemental is very strong in LSD (as I said on warlocks), but I believe it is pretty much the only viable comp os 3s.