I don't know how to tackle this question. There are a lot of movies that I enjoy, find entertaining and rewatch but would not consider the greatest ever. I find it difficult to watch really old movies and pretty much the only two black and white flicks I enjoyed are Nosferatu and Modern Times, and the later hade colored segments. A movie like 2001 a Space Odyssey have fantastic visuals, especially for its time, but it took me several times for me to actually view the whole thing since it bored me to death.
A few movies comes to mind, where they might not be the general top of the line but in their respective genres in what they try to archive they are top of the line. The Lord of the Rings still make any more recent fantasy epic look cheep some 20+ years later.
Requiem for a Dream gradually make the watcher feel more and more uncomfortable and is probably the best film I never want to watch again, not much is able to make me feel bad like that movie.
The strive and passion to reach new heights and the price that comes with it is done so fantastically in Whiplash that I forget every other film that explores that theme.
I guess the greatest film ever need to be great in every aspect movies have in common. Stunning visuals and cinematography, exceptional acting, an engaging story, a soundtrack with distinct emotional carry as well as being influential. And even if it is not a personal favorite and I think another movie did the gangster saga better, it is hard to ignore the master craft that is the Godfather.
Just for context, my favorite movie is The Prestige.
