Well before he lost to Cain I would have said Brock, and you could just close the book on it. However, I am curious to see how well he draws in his next fight. I expect it to be huge as always, but a part of me wonders if we will begin to see a decline in his drawing power. GSP is a good choice, probably the biggest draw outside of Brock, however he lacks the personality to get people really excited. Thank god he fights "bad" guys such as Dan Hardy, Matt Serra, and Josh Koscheck because they make his fights a lot more interesting.
A guy not mentioned so far is Fedor. He is a very big draw, because of the crazy streak he was on, but also because of how good he is and the fact that we rarely get to see him fight. I believe he would be an even better draw if he ever went to the UFC, but that is for another thread.