Bruce Lee was one of the greatest theoreticians of hand to hand combat and according to those who have met him also was a good fighter, but he didn't compete in a way nowadays athletes compete.
Concerning the greatest fighter, I'm with Budhisten. In my opinion, Anderson Silva is the greatest of those who have a reliable fighting proof. In my eyes he has definitely surpassed Emelianenko, as he regularly does things no other fighter can do. With Emelianenko it was always the same. Big overhand rights, Sambo takedowns and armbars and lots of decisions. Silva doesn't stop to find new ways to stop his opponents. I've written before, with him it's like he wants to prove that every martial arts technique that was ever invented can actually work in a fight if used in the right moment.