In the expansion, 4x Ret Paladin.
You could go with 1x Holy and 3x Ret, until then.

4x Frost DK's would work too.
4x Elemental Shaman can work.

4x Any Healer, is great in larger BG's.

4x Feral Druids
4x Enhance Shaman
4x Disc Priest (Smite spam, heals)
4x Shadow Priest

You have a lot more flexibility for battlegrounds and world pvp than for arena.
In arena, if you want to do well... you need a very optimal team.