Did I miss it?
Why hasn’t anybody said “The Princess Bride”?
It’s got “fencing, fighting, torture, revenge, giants, monsters, chases, escapes, true love, miracles…”
I’d find it hard to narrow it down even to a “Top 10”, and so much depends on one’s particular mood and preferences of the moment. And as much as I appreciate fine intellectual filmmaking, two great pieces of entertainment just in terms of pure movie-making craftsmanship that are right up there in my “most-rewatched” category are Jurassic Park and Titanic, the latter perhaps influenced by the fact that I’ve had a fascination with the subject since long before the wreck was even found, the former of course by the enduring fascination with dinosaurs. Titanic’s corny storyline is technically more than made up for by the painstaking attention to historical detail and terrific special effects. I seem to rewatch them every time I have some advance in my movie-watching technology, most recently in 1080p Blu-Ray with a new TV and my older but still terrific sound system. Next stop: 4K versions and a 4K projection system. 
Lots of incredibly great movies mentioned already, but my all-time favorite movie is David Lynch’s debut, Eraserhead.
Casablanca
I like you two. We should go down to the quarry and project old movies on the wall there. Maybe, “The 7th Voyage of Sinbad” to start. I have a soft spot for the Harryhousen movies of the 50’s & 60’s. Used to see them on weekend nights during the summer at the local Boy’s Club. In good weather we’d sit around the basketball court. In bad, the gym. They’d project them on the wall, old Godzilla movies, Three Stooges & Our Gang shorts, and the like.
Man, I can’t pick just one. Once upon a time, I would have said, “Blade Runner.” Later, “Local Hero.” I think certain movies mean more to you depending on when you see them, and what your life is like at the time.
Groundhog Day.
Because it’s about redemption.
Bullitt
What is ostensibly an action movie is actually a blend of late 60s American action/detective film with a contemporary modern Euro twist (thanks to Peter Yates and Philip D’Antoni). The city of San Francisco is the real star. Few if any sets were used. Excellent character acting…Robert Duvall, Simon Oakland, Jackie Bisset, Don Gordan and Norman Fell. And the absolutely on point score by Lalo Schiffrin. I never tire of it.
Its the perfect comedy/tragedy accentuated by the tragic death of its writer/director Steve Gordon at only 44.
Contact with Jodie Foster
Glad to see the love being shown for Bullitt; so much more than the famous car chase, and it’s certainly high on my all-time list.
Yes, most of my favorite movies have been mentioned too. And thanks for changing my #1 from “Maltese Falcon” to “TGTBTU”. Maybe. It’s very hard not to vote for Tuco. Also hard to vote against Sidney Greenstreet and Peter Lorre. Maybe we could get TCM to do a “Best of the Dope” festival?
BTW got to agree with Ukulele Ike about Victor Buono. Creepiest ever.
This a tough one. I was going to say Once Upon a Time in the West, since it’s my favorite western and I love westerns. But I’ve seen it so many times that it’s beginning to wear on me.
And then I thought The Big Lebowski might be my favorite. It’s definitely the funniest, IMO.
After thinking about it a bit more, I suppose my all-time favorite has to be Harold and Maude. Not only is it hilarious, but there’s just something magical about it. Perhaps it’s the time period (1971). I think I could watch it a thousand time and never get bored of it. 
My husband and I saw The Martian in 3D, on our wedding day. So it has special significance for us.
2001 A Space Odyssey. I’ve seen it 50 times at least, perhaps 100.
So many good ones and so cool that I’ve seen most of them mentioned and under different circumstance I could endorse any one of them … BUT, since 1979 my favorite movie is and has been The Warriors. I’ve seen it probably fifty times, easy. I’ve owned it on VHS, DVD, Blu-Ray, Director’s Cut, I’ve got the PS2 game of the movie and at one time I had the soundtrack on vinyl. It’s been a while since I’ve watched it but I could probably still recite the entire movie as it plays.
My brother and I say that to each other all the time! You are not alone.
Casablanca and Airplane! But the OP specified a single movie, so I’ll split the difference and go with Star Wars.
And when I say “Star Wars” you better know exactly which episode I’m talking about.
“Cover me, pawkins!”
The Sting, for the casting, the wardrobe, the backlot and the wonderfully implausible plot.
Definitely the best, though I’ve watched it only 19 times. (Of course I, just like Richard Blaine turns out to be, am a sentimentalist. Some with very different taste might watch Casablanca and be left wondering what the attraction is.)
Just like
Marilyn Monroe is the sexiest movie star ever.
Albert Einstein is the smartest scientist since Isaac Newton.
so
Casablanca is the greatest film ever.
is one of those trite-seeming clichés that happen to be true. Type “Why is Casablanca great?” into Youtube and get several hits of film experts explaining the great skills that went into telling this perfect story perfectly.