I've tried out Fire, Water, and Lightning putting 20 in each.

Fire was by far the most damaging element. I have up using the elemental weapon buff though and just starting chucking fireballs at everything. Gets even more crazy if you farm Flame Attack Power on every piece of gear. Most normal enemies die in one cast of the first fire spell.

Water spells were very weak, but it did make the bow hit fairly hard. So that's probably going to be top build for ranged, if that's even viable. Probably need the ammo return on every piece of gear to make that work.

Lightning was pretty terrible, even with most lightning attack power gear pieces. Was nowhere near fire damage. Only positive was getting access to some other buffs.

I'll probably try Earth next. Fire was by far my favorite though so far.