IMO Barao or Hendricks.
it is hard to vote for guys who will fight yet, because it highly depends HOW they win their fights.
Cain is also a strong candidate. but again, depending how he wins.
My vote for Barao is only because he had "to prove" he was worthy, and he beat great guys with ease.
My vote for Hendricks (if he wins) is because it is proven how hard it is to beat GSP. So any way he wins is impressive.
But quite frankly, I'd vote for Weidman, but as you said, with only one win this year, it is hard.
HOWEVER, if their next fight counts and he wins, he would get my vote. Doesn't even matter how he wins. because to beat Silva TWICE would be a feat any way you look at it!
Edit: Reading back the responses I am glad I am not the only one to mention Weidman, even if (like othesr pointed out) his fight would not count for 2013 (which is wrong IMO).