Favored Soul. Seriously.
Look in your group window; what are all the parties looking for? Clerics. Why? Because no other class can/will heal. You can argue all you want that "clerics aren't necessary, you just need to learn how to play", and then spout suggestions to spend a million plat on potions and wands, but most of us don't have that kind of cash, and even if we did, we'd rather spend it on sharp, pointy things.
Usually this discussion turns to druids, and if they were given healing abilities commensurate with clerics', that would be fine, but then you have two nearly identical classes. While I would like to see all of the base classes implemented, druid is towards the bottom of the list.
Another possibility would be to implement a better system for healing in-between battles (more healing clickies, maybe?), but I'd rather see the shaman.