The twins vs. the bullies in the nightclub in Stuck on You.
The Tick vs. The Tick.
yeah baby I’m the mad bomber that bombs at midnight
The Tick vs the laws of physics.
Must…defy…laws…of…physics. And the tick wins!
Punch Drunk Love Sandler vs those punks after they ram his car.
Happy Gilmore Sandler vs Bob Barker. And Bob Barker wins!
Well, after finally noticing the erroneous title of the OP, yours is the better choice since Jack was beating himself up and thus being masochistic.
I think you meant sadistic, Gad. 
This is exactly what first came to my mind as well.
For a completely opposite type of fight, drawn out and amusing, I’ll add John Wayne in The Quiet Man
Oops, not really one-sided.
Unforgiven when Clint cleans out the bar, especially the very end when he says.
‘Derserve got nothin to with it.’
The end ofTai Chi Master/Twin Warriors, when Jet Li has the unlocked the secrets of the Tao.
In Romeo Must Die (An underrated movie), Jet Li escaping from Aaliyah’s apartment.
A Bronx Tale. In the bar. “Now youse can’t leave.”
The most enduring beating in movie history is Patricia Arquette and James Gandolfini in True Romance. And both at various times get beaten badly.
The Bride taking out that veritable army of swordsmen in Kill Bill, Vol. 1. Damn but det was fun.
I prefer humorous beatdowns, so I have to agree with Darkhold and say the Bob Barker/Adam Sandler scene in Happy Gilmore. Also, the bathroom scene with Owen Wilson/Jim Carrey in The Cable Guy.
But, if we’re to take the thread title literally, I would say that my favorite is when Jim Carrey beats the crap out of himself in Me Myself & Irene.
Here’s a humorous one-sided beatdown I’m surprised hasn’t been mentioned yet:
The Championship Vinyl crew vs. Ian in High Fidelity.
When Dick rips the A/C off the wall to crack Ian’s skull, I actually did fall into the ailse laughing.
Paul Newman vs. Ted Cassidy in Butch Cassidy & the Sundance Kid.
“Rules? In a knife fight? No rules!”
Just a silly anime, but Those that Hunt Elves II has some of the most one sided combats ever. If you have never seen hardened warriors utterly defeated by an actress then this may be an education to you.
slaps you
looks at you dissaprovingly
slaps you again
Anyways, I’d have to say the scene between The Man with No Name and the gang in the beggining of A Fistfull of Dollar would be a nice addition to this list. I can’t help but get giddy every time I watch it.
Scene takes place after a gang scares The Man With No Name’s mule away by firing at it’s feet.
Man With No Name to coffin maker: Get three coffins ready.
rides over to gang
Gang member: Adios amigo, listen stranger, don’t you get the idea? We don’t like to see bad boys like you in town. Go get your mule. You let him get away from you? laughs
Man With No Name: See, that’s what I wanted to talk to you about. He’s feelin’ real bad.
Gang member: Hmm?
Man With No Name: My mule. He got all riled up when you fired those shots at his feet.
gang members start looking at each other
Gang member: Hey, are you makin’ some kind of joke?
Man With No Name: Mmm, No. I understand you were just playin’ around. But the mule-- He just doesn’t get it. Of course, if you were to all apologize.
gang laughs
Man With No Name: I don’t think it’s nice you laughin’
gang becomes quiet
Man with No Name: See, my mule don’t like people laughin’. He gets the crazy idea you’re laughin at him. Now if you apologize, like I know you’re going to…I might convince him that you really didn’t mean it.
gang reaches for their guns
Man With No Name kills all 4 members before any gang members can get off a shot
music starst playing
Man With no Name to coffin maker: My mistake, four coffins.
I also love the fight between Morpheus and the Agents and Twins in the freeway scene on The Matrix Reloaded, but that fits into the “guy has no chance, get into fight, and comes out victorious anyways” category. I still insist that Morpheus taking down that SUV was the most baddass scene ever 
.
DAMN YOU! You beat me to it. Nothing else even comes close!
ONLY 10 MORE DAYS TILL VOLUME 2!!!
Paul Newman in Cool Hand Luke. The beating he takes from George Kennedy is pretty bad, but he wins by refusing to quit.
Bruce Lee kicks about seventy-four different kinds of ass in Enter the Dragon, but the encounter that best fits this thread is when he takes out the dude with the scar over his eye. Lee is so fast that even when Scar Guy knows the fist will be coming at his face straight from the hand-to-hand starting position, he’s powerless to block or avoid it. That’s a one-sided beat-down for the top ten list, says I.
So far, the only suggestion that actually fits the thread title.
This doesn’t really count, since it’s not from a film, but I can’t resist–the Scotsman’s wife vs. the Master of the Hunt and his army in an episode of Samurai Jack. Jack and the Scotsman infiltrated the Castle of Boooooooone (don’t ask) to rescue the Scotsman’s wife, but were intercepted on the way out. The two mightiest warriors in the world face off against a seemingly endless horde and slowly get worn down. Soon it becomes clear that they’re not going to make it.
Then the Master of the Hunt makes the tactical error of referring to the Scotsman’s wife as “the fat one”, and she lets out a Xena-esque war cry and proceeds to crush everything in the castle with her bare hands, as her two would-be rescuers stare in shock.