Since he is a relative of mine, I will plug Buster's movie "The General" (1926) as a classic.
"El Cid" (1961) for historic drama and Sophia Loren in her prime (Hot!). Say what you will about Heston. It is good stuff regardless.
"Bonnie and Clyde" (1967) to see Warren Beatty and Faye Dunaway in their prime, under fantastic direction by Arthur Penn.
"Alien" (1979) Grab the edge of your seat and wait for Sigourney Weaver's money shot. Genius Ridley Scott directs.
"Unforgiven" (1992) Clint Eastwood's masterpiece, IMO.
"Glengarry Glen Ross" (1992) For Alec Baldwin's soliloquy and a fantastic cast graced with amazing writing.
For laughs: Anything by Mel Brooks will do in a pinch. (“It’s pronounced ‘Fronkensteen.’?†) And, of course, "Tommy Boy" (1995).
That's a few that come to mind
Wow a kin to Buster Keaton, that is cool.
The line from Unforgiven I always like is when Gene Hackman is dying and says he doesn't deserve to die like this and Clint's great quote, "deserve's got nothin to do with it." It's very true, things happen to us in life good and bad that we don't necessarily deserve.
Another thumbs up for Metropolis, forgot about that one.