Gunna have to go with Real Genius as my all-time favorite. Not far behind are Clerks and Mall Rats.
For "serious" movies, American Beauty, American History X, 2001: A Space Odyssey (even more amazing when you watch it and then Night of the Living Dead and realize the 2 movies were made in the same year)
Sci fi/fantasy... 12 Monkeys, The Princess Bride, the Star Wars trilogy...
there are tons of others, but I think these are my faves
1.1 BCI, 1.1 BRB, 0.1 Amazon Basin ETB