Meh, I do'nt care much for this list. AJ was hardly underused, and the other choices don't have much talent. Potential, perhaps, but not talent.
I would have to say that Matt Hardy would be my choice. While his brother Jeff finally looks to be breaking into the main-event scene, Matt has been stuck with midcard obscurity again. They never know what to do with Matt, and yet he gets some of the best pops on the Smackdown roster.
I go Matt Hardy.
Out of these choices, I have to say AJ, simply because he's the only one with talent.