I think the only one I've seen (or seen and remembered) is 12 Angry Men. It's one of my favourites.

I like the message, the way it's presented and the timelessness of it.

12 Angry Men is a hard one to top, a great movie that has been consistently kind of a let down when they try to remake it.

Beyond 12 Angry Men though, can't believe no one has even brought up Ben Hur or the 10 Commandments. Oh man, the original Strangers on a Train was in the 50s too. Lots of movies too that may not hold up in modern times but made entire genres or concepts viable. Notable shout out for Harvey and North by Northwest.

Akira Kurosawa is going to dominate this list.

Ikiru,
Seven Samurai,
The Hidden Fortress,

There's just no arguing that those are some of the best films of the 50s.

But there's also Ben Hur, Vertigo, 12 Angry Men, The 400 Blows, The Seventh Seal, North by Northwest, Strangers on a Train...

How do you choose?

having a list probably helps

https://www.imdb.com/list/ls062191077/

The only one I can think of offhand is Forbidden Planet. A great movie.

North by Northwest

The Day the Earth Stood Still.

Understated, but powerful early sci fi.

As a 40 year old who's not a cinephile, I'm going to say Abbott & Costello. Not sure which, haven't actually watched them since I was a kid, but maybe Meet the Mummy? I need to watch them all thanks for making me remember these comedy legends existed!

Best is a tough one. There are so many great movies in every decade. Narrowing it down to just one would be impossible.

To add to the post that others have supplied:

The Bridge on the River Kwai

An Affair to Remember

Roman Holiday

If you want to include animation,

Cinderella

Peter Pan

Lady and the Tramp

Sleeping Beauty

Sleeping Beauty in particular has some absolutely fantastic backgrounds created by Eyvind Earle.

If released in the 50s counts, my vote is for In a Lonely Place (it was made at the end of the 1949).

Adding “To Catch A Thief” & “Sabrina” to this list. Probably not as profound as some others mentioned here, but I consider them classics.

My highest rating on Letterboxd for a film from the 50s is The Night of the Hunter. I really haven't watched much from that decade. It is a pretty good movie, great at building suspense.