My favourites...

Instance grinding: 1 x Protadin, 2 x Mage, 1 x Hunter, 1 x Priest. I often swap the hunter for a lock.

That team is a complete nightmare in PvP though, where the pally (who is my "main") basically just stands there and does nothing useful at all.

BG PvP: 3 x mages, 1 x Warlock, 1 x Priest. BOOM!

I don't do arenas, and I live mostly on a concensual-only PvP server, so those are my two main activities

Of my newer teams, I'm REALLY enjoying the 3xBoomkin, 2xShammy setup with my wife, and almost - almost - wish we'd rolled them on a PvP server because it's the most well-rounded and capable group I can imagine. I miss crowd control at times, but with everybody basically able to take a beating, do ranged DPS AND heal, that's not a big deal.