And where would you put Anderson Sylvia and GSP?
I think that #3 thru #5 are actually very very close, but I have to give the nod to Penn because he has competed at so many different weight classes, and now he is settled in, for now at least, at 155 which I feel suits him the best. Like another poster said, the P4P list is one that is always changing so he could definitely fall down from this spot, but I feel that he has just been so dominant, lately.
#4 - I would have to give to GSP, the guy has beaten so many great fighters it's unreal. The fight versus Fitch should be awesome and then of course it looks like there is a good chance that he will fight BJ if he wins this fight.
#5 - Anderson Silva. The guy has been dominant in his weight class at middleweight. I think he is going to have some problems duplicating that dominance at 205 with the guys that they have up there, though. However, with about 2 solid wins at light heavy and you got to put him in the top 3.
Urijah Faber - Maybe I am overrating Urijah Faber at Featherweight, but I think his punching power at this class is devastating( he doesn't need to throw combos to KAYO opponents, he can put fighters on their ass with one punch, ala Rampage.
Of course you are always going to have the world class conditioning with him, the punches in round 5 are going to have just about as much sting as in round 1. His defensive style stance and movement is very hard for the opposing fighter to penetrate.
The reason I put Fedor #1 and Faber #2 is that I just can't see them losing anytime soon at their respective weight classes.
This is the beauty of a P4P debate, everyone has their different beliefs on what should constitute it.
I also believe that these are the top 5 fighters in MMA as of today, and the quality does drop off a little after these guys.