I think it'd be The Dark Knight, just cause it's what I'd been waiting for as a comic book fan for my life so far. It followed the comics perfectly (while still being respectable as a movie) had a great cast, great acting, action, cinematography, etc blah.
I acknowledge it's faults like the over hype it got, Batman using witty quips, using a computer to deepen Batman's voice to horrible levels, over doing the plot from a movie standpoint.
But for me, the movie was right in what it did. It appeased the kind of fan I was. Who went to see a movie, but wanted something very close ans true to the comics.
For the record, I think Batman Begins would be a better "film" in terms of character development, plot, and the like.
Alien was fantastic. I'm not into horror/thriller/slasher/whatever, but it was very entertaining as was the second one.
Princess Mononoke is another one I love watching too. I'd think Spirited Away is the best Miyazaki movie, but Mononoke Hime is more fun to watch. (First watched Mononoke Hime when I was 7, not a smart idea.)
Monsters Inc. is up there being my favorite Pixar movie, though I do like Cars quite a bit as well.
That was quite a long post from someone who doesn't watch movies very often, but I felt all those movies deserved mention.