Everyone loses a fight if they stay in the game of MMA long enough.  This is a sport of match-ups and styles.  I would like to see your number one ranked fighter...pound for pound...not by weight class.  This is a difficult, if not impossible, thing to prove with the weight class system so just have fun with this one.  Here I go:

Fedor has unbelievable balance, great striking on his feet and on the ground, as well as unbelievable submissions.  He is my pick for number one.  He has lost one fight and it was due to a stoppage from an illegal headbutt.  Because it was an elimination tournament...he had to be given the loss so that their could be a tournament champion.