Wow. So many options. There can't be only one... And speaking of which:
Sci-Fi:
Highlander (only the first. None of the rest qualify as anything except for a way to steal money from fans of it)
Dark City (Rules)
Matrix (with the same rules as the first Highlander. The rest are an abomination)
Dune (Extended Original) The remake for the Sci Fi channel was actually really really good, too.
THE FIFTH ELEMENT (campy sci fi at its best)
There are many more, but ....
Comedy:
Kentucky Fried Movie
Kung Pow (Enter the Fist)
Princess Bride
Monty Python's Holy Grail
Again, too many more to list.
Action/Drama:
Silence Of The lambs
Jaws
The Professional (Leon) This movie is epic. Gary Oldman is THE man
American Psycho (c'mon.. what dude, doesn't have a man crush on Bale ?)
The ENTIRE BOURNE SERIES (never read the novels, but amazing movies IMO)
Random:
Fight Club
The Right Stuff
Gladiator
Reservoir Dogs
he11, let's just say mostly anything involving Quentin T.
Oh.. lastly.. For those of you who really like Christian Bale (though to be honest, I don't think that Batman does much for him. I hate that stupid voice he does.. sounds like a cranky old Clint Eastwood.. it's annoying and contrived) if you have not seen "The Machinist" I highly recommend it. He really put himself through real physical trauma in order to fulfill the roll he played, and was a great/creepy movie.
Doug