Like everyone else here, I feel it’s an impossible question. The answer has changed over time and even with mood. These are films you can watch over and over again and still find them entertaining. I’m with @zenvelo on Casablanca. Every time I see it, I find a new laugh, or a new wrinkle.
The Sting is one of those. Love those old grifter stories when nobody experiences real bodily harm. This is a good one based on the exploits of one of the greatest American conmen of all time.
A couple of years ago, I dug up a sweet old film I watch a couple of times a year. Charles Laughton and his wife Elsa Lanchester in The Beachcomber, 1938.
A lot of Frank Capra’s work falls into this category.
I never tire of watching Chinatown, 1974 with Jack Nicholson. Greatl color, cars, theme music and period dress. I like LA in the late ‘30s through the early ‘50s. And I like to watch the sequel, The Two Jakes afterwards.
Then there are all those Noir classics that I can watch over and over. The Big Sleep is complicated—even author Chandler couldn’t tell the producers who actually committed the murder when they wrapped. Every time I watch it, I find more clues. And all the code and euphemisms they had to use for the types of sex and sexual orientation described in the book. They got in real trouble for mentioning “the Singapore Sling” in that film. The censors were all over them during filming and that one got through. It’s fun to pick these things out. New stuff every time.