I couldn't think of a single greatest so (as I enjoy making lists a little too much

Add more genres or whatever. I admit to being in a quandry over adding an SFF option but the good ones are all "action movies" imo and some of the ones I've listed are open to interpretation. So, whatever, just give me lists and show off your superior list making forum fu.
For me (yes I have alphabetised by genre

Action - Aliens
Animated -
Chick flick - When Harry Met Sally
Childrens - Ghostbusters or Bill & Ted's Excellent Adventure
Comedy - Some Like It Hot
Crime - LA Confidential
Foreign language (non-English) - Pan's Labyrinth or Cinema Paradiso
Guilty pleasure (I don't mean porn!) - Kindergarten Cop or Kickboxer

Indy -
Superhero/comic book - Batman Begins (yes I have seen TDK)
Western -
War -