Shaman hands down. But besides just saying that, let me explain briefly (and I'm probably grossly over-simplifying).
A cleric or druid will help keep you alive better than a shaman in terms of healing power.
A druid will help get you out of bad situations and ease the stress of hoofing it long distances (evac, roots, ports and the like)
A shaman will make you do a TON more damage and is probably the best CC with VP.
Having the shaman will bring the most DPS, so you can kill more mobs at a faster pace. The healing advantage that clerics and druids have will essentially be negated by the shaman's slowing ability. Also, choosing a duo partner based on bad situations that you MIGHT run into, or simply for convenience probably isnt the best idea. Also, you can get cleric buffs that last a TON longer than shaman buffs. And as a melee DPS class, shaman buffs help quite a bit so it is more useful to have a shaman around at all times rather than a cleric (in terms of buffage).
Anyway, back to work. If anyone wants to give better detail or tell me I'm wrong in an aspect, please do.
"Sometimes you have to face your demons, sometimes the demon is you." - Yesak Demunstorm