Shamans are an obvious choice and I would say that 90% of the 5+ boxers I have come across use mainly shamans, sometimes with another class thrown in.

Diversity is what we need! I'm planning on 5 warlocks (4 Destro & 1 Demon), 2 Druids (1 Balance & 1 Resto), 1 Mage, 1 Paladin (Holy) and 1 Priest (Disc).