I think that frost is pretty over powered at the moment - burst damage (dropping 65k damage on a deep freeze target practically instantly) coupled with survivability. Fire is viable as well with moving scorches etc but I'd still reco. frost. Air on the side of personal preference.
The current trees are fairly forgiving. You can optimize for whatever you feel is best for your play style.
[us.battle.net/wow/en/character/black-dragonflight/gravitation/simple]