The person who will get closest to beating Fedor right now is probably Barnett. I was extremely impressed by his performance against Monson.
I second this one, Barnett looked really impressive in the 3rd round of the Monson fight. Although I also want to say I think Monson could very well give Fedor a few problems as well as Monson's striking seems to be somewhat improved as he managed to rock Barnette in the first.
If the circumstances were right I think Randy could beat Fedor but unless the UFC signs Fedor I think Randy gets beat down in a ring and I dont want to see that, I actually think Tim Sylvia may have as good of a chance as anyone as he is such a unique combination of size an skill that it is hard to say how Fedor will handle him, Fedor has fought much bigger and much more skilled fighters though so its kinda a toss up.