My opinions
PVP/PVE:
3+ mage, resto druid/pally
3+ hunter, resto druid/pally
4x warlock, priest

PVP exclusive:
3+ shaman, pally
5x shaman (1s resto)

An interesting combo imo would be

2x hunter/2x felguard warlock/resto druid or shaman


There is no best team... it's all personal preference.

I mean some people run a tank and 3 casters and a healer (normally warrior, 3x shaman/mage, pally)