The Weapon XI fight is probably the best one-on-one fight imo (well, one-on-one when he fights Wolverine, that is). Wolverine vs. Gambit could have been a contender if it was long enough (Jeez! Talk about not knowing how to draw out a good scene - they really dumbed down Gambit's fighting skills in that!). I would say that the overall best fight scene in all of the movies was Nightcrawler's infiltration of the White House.
Awesome sauce!