Ones that added something new that wasn’t seen before, OR that revolutionized something, OR took something that’s already been done, but made it fun anyways.
That could be anything; concept, stories, imagery, execution. A lot of movies are classics for different reasons. I don’t think a movie needs to be old to become a classic, but seeing as many old movies introduced something new first, they get that respect. I mean even back then, so much shit was all the same.
I love old black and white horror, and obviously, as I’m sure you know, everyone wanted a piece of the Dracula pie, for example. They’re not all good, even if they’re old. But I enjoy them all.
But one comes along and becomes a classic anyway; look at Dracula Love Never Dies. That was from 1992, and it’s considered a classic by now.
Well I suppose I could go on with all my indefinite answers…
TLDR; a movie becomes a classic probably just by how much money it makes haha.