I couldn't narrow it down to just one. Comedy wise I can watch Monty Python and and the Holy Grail over and over and find something new and funny in it. It's alot more than the black knight sceen which is sooo classic. "Its just a flesh wound" ... "I've had worse".

Drama ummm Natural Born Killers would be near the top if not on top. It's really more about modern society than 2 killers IMO.

Good Fellas or Godfather 2 for gangster stuff.

IMDB dot com puts Shawshank Redemption at #1 all time. I disagree with that, but it is very good.