In my experience, I've found that the easiest classes to get maximum dps from are demonology warlocks, hunters, and cat druids.

If I ever get everyone to level 85, I'm looking forward to running the following groups in BGs to raise hell:
5x demon locks
5x shadow priests
5x frost mages
4x rogues + druid
4x hunters + priest
4x dks + priest
4x enh shaman + druid

Added: I recently read somewhere that shadow priests are currently the highest DPS class in the game. I don't remember if it was pve or pvp or even where I read it...