I've been streaming The Hindenburg this evening, and it holds up really well. I don't just mean for a movie made in 1975, I mean period. It helps to know a little something about history, modern audiences might be put off by not knowing any of the actors, and there might be one or two effects that could use a nip or tuck. Beyond that, you could probably run this in a theater today and have it do okay.
I've sometimes thought that even with its few plot holes Raiders of the Lost Arc stands up pretty well too, give or take one or two effects in the final action scene.
What other older films do you think stand up pretty well?