It’s McClintock!. One of the Duke’s better films. A fun fight scene, and Wayne seriously mocking his own image.
Naw, the most powerful punch ever shown in American Cinema occurred during the Godfather, when Sonny is beating the crap out of his brother-in-law, Carlo. One of Sonny’s punches is so strong, it rocked Carlo’s head back even though it is clearly evident that the punched missed Carlo by a good 5 inches.
Man, that Sonny Corleone is a pisser!
To be fair, it was the nut shot that won the “knife fight” in Butch Cassidy. The roundhouse punch was just the mop up move… 
Paul Newman is soooo cute…
There’s a great punch near the beginning of Jailhouse Rock where Elvis is in a bar brawl and some dude decks him. When his head snaps back from the force of the punch, his hair is all disheveled and crazy, but a second later when his head whips back upright again all the hair snaps back into place. Hilarious! Elvis then nails the guy back and he dies when he crashes into the jukebox. That and King Creole are the only two good Elvis movies that I know of.
Story of Ricky (Ricky Oh). Nuff said. 
Hey if we’re talking about the Duke, I’ve always loved the fight scenes in The Quiet Man. Especially the part where they take a break to have a drink, and then get into it again and one of them goes flying through the tavern door. It’s just so rowdy and silly.
There was a good punch in Victor/Victoria where Victoria is hiding in a wardrobe, and when the door is opened her fist comes flying out, and knocks the guy out cold.
Of course that doesn’t compare to some of the others already mentioned, especiallyBlazing Saddles, but it’s what I thought of.
When Ripley tells the alien, “Get away from her, you BITCH!” and then knocks the alien across the room that forklift arm.
Love that punch.
Fairblue is close - it’s the punch we didn’t see. McLaglen had just throw a drink in Wayne’s face.
::Wayne squinting: “Bar Towel”
::barman supplies it::
Wayne: “What time is it?”
barman replies
Wayne cocks fist while McLaglen is facing forward drinking his pint.
Camera shot of outside of bar. McLaglen comes thru door, shattering it. Fight over.
Natural Born Killers. Can’t remember the lead actresses name - she was also in Cape Fear. I have NEVER seen a woman throw a punch like that. She is a natural born puncher if I have ever seen one.
Dammit! I wanted to say that one. I like when he punches right through the guy.
That was Juliette Lewis in Natural Born Killers. A little lady with a psychotic streak a mile wide.
It’s not exactly a great punch, but I like a scene in Silkwood in which Kurt Russell sucker-punches Craig T. Nelson. It’s realistic in two ways: Nelson is knocked down but not unconscious, and Russell doesn’t hang around for him to get back up, but makes tracks out of there.
Of course, one might mention the gut punch Schwarzenegger puts all the way through Bill Paxton in The Terminator. You don’t really need a lot of other moves if you can do that.
I thought of two. One is the Mongo/horse punch. The other is in *Seven Brides for Seven Brothers, * when Gideon, the youngest brother, punches out Adam, the oldest brother. Gideon was mad because Adam had made a couple of disparaging cracks about Adam’s wife, and refused to go see his new daughter. Gideon is such a kid, but he says, “I wouldn’t count myself no kind of a man unless I did this.”
That punch in Snatch was really good.
There’s a scene in Strange Days where Ralph Fiennes gets past an NFL player turned bodyguard by beginning a conversation then decking him really hard. I liked that one because I didn’t expect it so soon after he started chatting, and because it was surprisingly effective.
Also, Indiana Jones/Last Crusade. The “Scottish Lord” scene. God I love that little sequence. I have it memorized, even. Not surprising, though, since I have most of the movie memorzied.