Brock Lesnar: The HW division is growing very quickly and their are many dangerous up and comers that can challenge Brock greatly.
Lyoto Machida: The LHW division is arguably the most stacked division in all of MMA. There is a lot of top talent here that Machida has yet to face and the top fighters are all extremely dangerous fighters. His upcoming fight with Shogun will prove another tough test for Machida.
GSP:St. Pierre has dominated most of the top fighters, but fighters like Koscheck and Fitch have improved since fighting GSP. There are also fighters like Paul Daley and Dan Hardy who have the KO power to catch GSP at any time.
BJ Penn Penn losing the title is very doubtful but the LW division has some very good fighters that can threaten Penn. I think that Florian will have a much different gameplan if they meet again and we can't forget that Gomi wants a shot at Penn as well.
Anderson Silva: I don't think Silva is the most well rounded champion in the UFC, but I think that the MW division is the weakest division right now. Silva has rarely been in trouble and if he gets past Maia, I am not sure who will take him out. I think that Vitor is his toughest test and with Silva fighting soon and Vitor injured, I am not sure when they will meet.