Air is great, cheap damage and the incredibly abusable Cyclone. The only drawback is that quite a few enemies have pretty high resistance, still easily worth it though. I also heard Prime is pretty good, and you should take it to Master just for the utility anyway. Not a huge fan of Dark, bosses have too many immunities -- Purge is absolutely essential though (IMHO), but it's just expert level.
I'm probably going with Air (certainly, already have it up just missing the GM since I still need the promotion quest) and Prime.