Shockingly I would choose air. Mainly cause you can fly which is awesome.
You could fly with most elements. You could bend the ice to go under your feet and slide around everywhere and pretty much the same for earth. Fire might be a bit harder. At the beginning of the show, you see Avatar Roku bending lava, which really doesn't make any sense. Anyway, you could bend that under your feet, too. If firebenders can bend lava, anyway.