1956 - Forbidden Planet (great sci-fi concept and earliest appearance of Leslie Nielsen I'm aware of)
1979 - Alien (sci-fi-horror fusion, strong female lead)
1984 - Gremlins (timeless practical effects, poorly-aged Chinese character)
1985 - Back to the Future (does it need a blurb?)
1994 - Dumb and Dumber (classic Jim Carrey, very quotable)
2000 - O Brother Where Art Thou (Coen brothers, Americana, great soundtrack)
2006 - Idiocracy (goofy concept, now a documentary)