easiest is lightning. combos well with all. water is powerful with poison. when teamed up with another poison user can be really strong. fire is most damage overall, top world boss dmg is usually fire. wind is great especially if you choose to spend at least a dollar for gaara, thats what i have now. gaara is worth it anyway. even earth is good with interupt. its all about what type of team you are looking to build. i would say lightning works best with team 7 at least early on.

Fire sound appealing now :)

All in all fire and lightning look are dominating this game after all.