The original House on Haunted Hill (75 min)
Also Primer (77 min), one of my favorite movies ever, which is great because you basically want to watch it start to finish again right after seeing it.
Lots of anime and animation comes in just under 90 just because it's expensive, although I don't think that fits the spirit of what you're looking for. But I mean, I'd say Toy Story is an all time classic under 90.
Edited: 10/14/2019
at 12:33 PM
by DefaultGen