I too also have to break it down by genre, although Star Wars can remain at the top as an overall winner.
Sci-Fi:
Star Wars, the first one (Episode IV)
I was just a kid but nothing has since rivaled the jaw dropping experience it gave me back then.
Action:
Raiders of the Lost Arc (much the same as above)
Fantasy:
Lord of the Rings (entire trilogy)
Comedy:
Monty Python and the holy Grail
War:
The longest Day (maybe because I just watched it again lately on TV)
Horror:
The Exorcist (The only horror movie that can still make me queasy.)
Not sure where it fits:
The Princess Bride (As Jake mentioned, i just has to be somewhere.)
Can't bring mself to really pick a favorite drama. There are just too many movies made in this category I can't pick one.
My not so secret indulgence:
Highlander, Only #1. The rest should never have been made.
