I currently have 3 reasonably geared teams at 80 - Shams, Druids & DK's. I'm also levelling a Pally team (non-RAF, oh the pain...THE PAIN!).

Quick summary on each:

Shamans - best for PvP, but need to drop one for a tank to do heroics. Still my favourite team to play...will be even better in PvP with the next patch.
Druids - The all-round team, good for pvp, good for heroics, but not greath at either. Lots of fun to play, but have a few drawbacks - having to shift to heal and vulnerble to AoE fear.
Death Knights - good for pvp (esp with glyphed rune tap), can do some heroics, but lack of heals is a drawback.
Paladins - shaping up as a solid choice for both pvp and pve. Complementary auras and blessings, heals and wipe prevention, and a range of stuns make them an attractive choice.

Hope this helps...

Cheers,
S.