Had a pretty good think about this myself, being such a big fan of film in general its really hard to put just one in that title! So I put 3 instead, which turned out to be a little easier!.
IMDb - Aliens Definetly one that I just can't get tired of. Sci-fi action at its best, that still manages to keep up with the times. Absolutely timeless!
IMDb - Rocky Horror Picture Show Egh, not a fan of this film at all... But having two little sisters that put this on at least once a day as kids its up there as a most watched film.
IMDb - Dogma I make a habit of watching this film at least once a week! Razor sharp dialogue, creative story and lots of sick humour! My perfect film, without a doubt!
You're absolutely right about the question Gogo, definetly a toughie!
Cheers
Adam