For PVE, just about anything works. Just figure out what utility you want in your group the most.

Better AoE heals? Priest, then Druid/shaman
Better single-target heals? Druid/Pally/Priest/Shaman
Want curse removal ability but no disease? Druid
Want debuff dispels except for curses? Pally
Want passive cure poison/disease? Shaman
Want anti-fear/sleep + passive AoE heals + passive AoE damage? Shaman
Want +DMG for caster DPS? Shaman
Want in-combat rez? Druid
Want wipe prevention? Shaman
Want usable healer threat reduction? Pally/Shaman
etc.

Pick what you want and go with it. With the way gearing is in WotLK, any of them do well.