Obviously, have not seen all westerns, but 2 scenes stand out to me.
The first is not a fight, but a shot. Burt Lancaster as Bob Valdez in 'Valdez Is Coming', sitting atop a hill, and making handloads for his Sharps buffalo rifle. The bad guys point man rounds an outcropping, and Valdez shoots him off his horse at 1000 yards. Because the landscape is very open, the shots sound like a jet fighter zooming by.
IMHO arguably the best western gunfight is the big one in 'Open Range'. They got 99% of that right with very very little Hollywood gunfight silliness.
I'm a big fan of the slow-paced gunfight in My Darling Clementine. It's right behind Stagecoach and Unforgiven as one of my favorite Westerns. The Gunfight at the OK Corral is a fitting climax. Victor Mature was great. So was Walter Brennan. It's my best impression from when I was a kid and a weekend Country jock. "One of these days I'm gonna climb that mountain." Always had the old boys laughing with that one. Haven't heard "Old Rivers" on the radio in decades, I don't think. I digress...
Glad you posted this. I wanted to make a couple specific points about the fight.
1. There are 2 things some people would fault the fight for. One is the shotgun blast that knocks a bad guy 10 feet through the air. Sir Issac Newton laughs every time that is shown.
2. The 10-shot Colt Peacemaker revolver. Though usually a major fau pax in westerns, the commentary track states that the scene is a deliberate tribute to old westerns.
3. One scene completely blows my mind. That is the hostage situation in the alleyway, when a bad guy is using Annette Benning as a shield. This is where we see what a total badass Costner really is. He has a Colt in each hand, and simply advances on the situation, firing alternate hands, and installs some serious ventilation in the bad guy, even giving a couple 'make-sure-he's-dead shots after the man is down.
I have had some fairly serious training in self-defense handgun shooting and that right there is pretty amazing. What is shows is complete confidence in his abilities, and some really good tactics. He doesn't stand still long (shoot/move), and though his rage is palpable, he has complete control over it. That moment is 200% badass...