Cousins would have been the most beneficial for the 2010-11 season, since we were thin up front. He gives you another legit go to scorer, so we would not be as reliant on Knight.
Knight is the only option for the following year since he was our only one and done. His addition would be pointless, since we obtained the ultimate prize without him. I don't know how he would mesh with that team, since he would no longer have free reign to do what he wanted. The potential on offense would be exciting.
I don't think any one player makes that much of a difference for that 2012-2013 season. It might have kept us from going to the NIT, but we needed more than one player for us to have a prayer of actually doing anything in the tournament. Even if all the freshman come back, that team still has obvious flaws.
Noel is the best option for 2014. A better WCS basically.
I don't think Young or Randle significantly improve last year's team. Randle would just add to the problem of finding minutes for all our frontcourt players. Young would have allowed us to continue to platoon, but he would need to significantly improve in several areas for him to be a better option than what we had.
This year, as others have said, Towns would be really nice. Having a guy who is a true center and can play physically down there, with his talent, would be perfect. Then the only thing we would be missing is a true small forward.
I guess my answer would be Cousins or Towns.