Schindler's List hits one of my very top spots and it seems like sacrilege to even acknowledge another movie in the same sentence, so I'll start a new one.
Since we own over 350 movies I have to break it up by Genre to even start...
Comedy : Chasing Amy, Kevin Smith's Best movie, although I like all the others so far...
Horror/Thriller : Se7en, with honorable mentions to Donnie Darko, The 6th Sense and Stir of Echoes
Drama : American Beauty and LA Confidential are high up here but there are so many it's hard to decide.
Fantasy : LOTR : ROTK, followed closely by the other two LOTRs
SciFi : Empire Strikes Back, with honorable mentions to The Matrix, Serenity, the rest of the Star Wars movies (minus Eps 1 and 2)
Western : Unforgiven, Tombstone is good as well.
Action : Any of the Indiana Jones movies...
Family : Anything from Pixar...
Unclassifiable : Pirates of the Caribbean : Curse of the Black Pearl, comedy?, Action?, Drama?, Family?..... can never really decide where to file this one....