I don’t channel surf all that often in the age of DVR and internet. But when I do, there are a number of movies that can make me stop. I have a hard time picking just one, but the most likely to suck me in is probably The Blues Brothers. All the great musical sequences - Ray’s Music Exchange, Bob’s Country Bunker, Aretha Franklin fer gawd’s sake - “Don’t you blaspheme in here!” Oh yeah.
The Longest Day. Most awesome cast, ever.
Classic Hollywood comedy doesn’t get much better.
Some great choices already, but I always stop for *The Mummy *(Brendan Fraser version). Lots of pretty people, and great tongue-in-cheek humor.
“Look what I’ve got!”
meow
Well, I would have said The Fifth Element, but just to be different - Big Trouble in Little China.
Armageddon.
Any of the Bourne series.
This is mine, too – it happened just last night, as a matter of fact.
I was a pretty disillusioned kid growing up, but Harold and Maude snapped me out of that funk. Stops me dead in my tracks every time.
“What gives you that… special satisfaction?”
“…I go to funerals.”
Snatch
Godzilla (1956 original, not cheezy-sleezy remake).
Willy Wonka and the Chocolate Factory (Gene Wilder version)
What’s particularly amusing about this thread are the number of responses that are movies that haven’t aired on tv for decades (to the best of my knowledge), or if they get aired it might be once in a blue moon. I mean, it’s right there in the thread title! How can a movie qualify as one that you would “always stop channel surfing for” if it is never on tv?
I’m gonna go with the greatest bad movie of all time, Point Break.
Damn!!!
Too. Many. Good. Choices.
But if there’s only ONE choice, I think this would be it. Don’t we get a second, maybe a third choice? doesn’t matter I’m TAKIn’ one. 
Usually one of the Molly Ringwald three will do it.
Night of the Comet.
Stardust.
Funny, considering your user name. Since you reminded me, if they ever put it on TV, “Local Hero” might override “Road House”. Interesting, in that they’re polar opposites. It’s such a great movie, really immerses you in it’s offbeat, quaint, quirky universe.
“You’re a shit, Happer! Craphound!”
“How do you do business with a man who has no door?” “The ethics are the same.”
“You’d like to buy my church?” “Not as a going concern. . .”
And possibly my favorite scene, “I’d make a good Gordon, Gordon.”
Great 80’s movie!
“We’re gonna play a game I call, ‘Scary Noises.’” “Oooh, an E-ticket attraction!”
“Daddy would’ve gotten us Uzis.”
The Godfather is the main one for me. But there are others that are close and I’m going to mention them regardless of the “one movie” rule:
Any 80’s John Hughes movie.
Office Space.
Trainspotting.
Any Lonesome Dove series except the crappy, not based on one of the books one.
God help me… 
Independence Day
:o
I’ve got a dozen that I will stop on no matter how many times I’ve seen them. Most of them have been chosen by others already. Notably absent is Citizen Kane, which will be my contribution.