I never really know what "old movie" means, like some people consider the Scarface remake to be an old movie by this point (and I guess 30 years is kinda old) but a lot of people don't. So I don't know.
I really liked Chinatown, I often feel like I like neo-noir more than classic noir. I think at least part of it is that filming technology during the classic noir era made low-light/night scenes looks pretty terrible and that's one of the thematic staples of the genre.
I watched 12 Angry Men fairly recently and was blown away by how well executed it was despite is age. I think the simplicity of its premise and set has really helped it age well.