Got to be the fight between Chuck Norris and Bruce Lee at the Colisseum in Rome. I forgot the name of the film but it is a really bad film and Norris plays an American karate champion hired by a gang boss when he realizes that his thugs can't beat Lee.
The scene in Fight Club when Lou interrupts the fights going on in the basement of his diner. Tyler Durden (Pitt) takes slug after slug and just laughs. It's the way the fight ends that makes it the standout fight for me. Lou has fallen but Tyler has a hold of his belt while his thugs are trying to pull the heavily bleeding Durden off their boss. He screams "You don't know where I've been Lou ... You don't know where I've been." Of all the fights in the film, this one is most emblematic of the story as a whole.