Maybe I'm a sucker for big budget bad-ass somewhat realistic fight scenes, but.....
In a general, yet no particular order.
Lethal Weapon 1 - Riggs vs. Mr. Joshua (Gibson vs. Busey) on Murtaugh's front lawn after they drive a police car into his house. Spotlight provided by police chopper, 8 foot metal pole vs. billy club with a broken fire hydrant drenching them both?!?! DAMN!!!
Snatch - Mickey O'Neil vs. large bare knuckle boxer is last fight scene. Mickey (Brad Pitt) is fresh off a drinking binge at the wake of his mother, gets the crap kicked out of him for 6 or 7 rounds before being flattened by an uppercut. "All he has to do is stay down." He gets up and lays out the guy twice his size with one punch, dooming his tribe of Pikies, who had the turn on Brick Top the whole time.
Fight Club - The Narrator (Ed Norton) vs. Angel Face. This is the more gruesome beat down I've ever seen in a movie. Angel Face's face is just hard to look at after Norton is done with him. A close 2nd in this movie is when the bar owner comes down and beats the piss out of Pitt while Brad is just laughing at him.
The Matrix - take your pick, lots of great fights in all 3, but I personally like the ones in the first movie better as I think they did a lot of movie fight scenes by introducing a lot of new filming techniques.
Happy Gilmore - Bob Barker vs. Happy. Do I really need to explain any further?
Saving Private Ryan - Pvt. Stanley Mellish vs. German foot soldier. You know you were cringing in your set when the germen pulled out his knife and slowly was inching it down toward Adam Goldberg's chest. Yes, short scene, but very much to the point, and was a a key part to one of the underlying story lines in the movie.
The Princess Bride - yeah, yeah, the same one. Inigo Montoya vs. the Man in Black. Great sword fight scene for it's time, even better humor through that entire scene. "Why are you smiling?" "Cause I know something you do not, I am not left-handed."
Indiana Jones - Raiders of the Lost Arc - Indy vs. large bald Nazi. Amazing fight scene for 1981. Dude getting chopped up by the propeller of a WWII prototype plane is priceless.
Monty Python and the Holy Grail - King Arther vs. Black Knight. "Look, you stupid Bastard. You've got no arms left."
Rocky - Rocky vs. Creed. The one that started it all (and unfortunately, it never stopped).