I've tried this exercise many times and cannot settle on one film. I have considered some the very best of their era (Citizen Kane, 2001: A Space Odyssey) but modern audiences don't get and then a I see a film which really touches something deep and although it might not win awards, for me it has done the job of reaching into me (Collateral Beauty, Bridge of Spies). Then there are the ones I just watch over and over as a kind of 'safety blanket' when I'm feeling too tired to use my brain.
I would put Rear Window in the great films of the 60s (and definitely in the top work ever in cinema history) and Vertigo is wonderful, but when viewed from 2020 the way women are objectified and pushed around in the narrative makes uncomfortable watching.
Interstellar has to be a modern sci-fi classic and the legacy of 2001 can be seen in it.
Fortunately, as in the famous art galleries, there are many masterpieces.