I find that hybrids really have too much downtime for mana, so while I like the concept/eye candy of shaman/druid teams, I won't bother with them yet. Priest + mages/locks seems to be the easiest to be successful with quickly. Priest + hunters is a good mix but the limitations in enclosed spaces makes it dicey and very frustrating at times. Balanced groups are good for static encounters but fall apart when the shit hits the fan. Melee groups would be awesome but keeping everyone facing the right direction and in melee range on multi-mob pulls and especially PVP is a nightmare. 5 shadow priests is awesome but breaks down in the same spots as the other groups when a real tank is needed.

So, in short, it kinda depends on what you want to do with your group. Probably the most versatile group I've found is the priest+warlocks so far. Fear kiting for hard stuff, pets for ghetto tanking in some cases, DoTs that can be applied en masse while both moving and not facing the target, unlimited mana (more or less), good AoE, good PVP survivability and damage, good pets for annoying/debuffing/CC, etc. It's a good all-around group.