Sham / druid / lock. in that order.

I also think you'll have the fewest 'OMFG that was sick' moments with shaman.
Druids have starfall + trees = pain.
Warlock: 4x Seed of corruption madness + 4 infernals + 4x Metamorphosis will be quite deadly aswell.
Shaman just do the job with focussed nukes. Allthough it might take a while untill oneshotting people gets boring