Middleweight doesn't have much competition for Silva. Silva is at the top, then there is everyone else. I think Hendo has the best chance beating Silva, but he won't get a re match until he get a couple wins at MW which will take some time. I think the best battles at 185lbs are the guys fighting to be the #1 contender, not the title fights.
Welterweight IMO is the deepest division in the UFC. GSP is going to have tough battles in each title defense, but if he pulls off the same performance as his last two fights no one at 170lbs can touch him. He dictates the pace of the fight and determine where the fight is going to be. That's a nice advantage to have.
Heavyweight, I don't see many guys in the UFC taking out Nog, but there two guys that can easily sign with the UFC and win the title: Fedor, Barnet.
This is a tough call to make