Benson is unacceptable to me because he should be at the very least 2-1 this year, he lost the second fight to Frankie, and I thought he lost the first as well.
Jones and Anderson weren't active enough against contenders to warrant it. Anderson starched Bonnar, but he's a mid-tier LHW, and Silva's win over Sonnen was marred with a bit of controversy. Jones had a dominant, but lukewarm win over Rashad and then almost got submitted by Vitor Belfort, a mid-tier (imo) MW.
Rousey's competition is not good enough to be considered for this, same goes for Cung Le.
Weidman is without a doubt, the breakout fighter of the year, but not the overall fighter of the year. This year he shed his prospect label, and got two dominant wins over top 10 opponents, one of whom is now top 10 in another division.
Hendricks has had a great run this year, but his win over Koscheck was questionable in a lot of people's eyes, so I don't think it warrants Fighter of the Year.
Cain is a definite candidate, with two dominant wins, including avenging his title loss and getting his belt back.
So in my opinion, it's between Cain and Mighty Mouse, and I give it to Mighty Mouse. He goes from being a top 10 BW last year, to beating the former number 1 Flyweight TWICE (yes, twice, he won the first match IMO), and then beat uncrowned king of the Flyweight division, Joseph Benavidez, from pillar to post to take the belt and win the UFC's first tournament in years.