I didn't like either of these movies the first time, but The Fountain and Unforgiven are up there for me. The Fountain is really profound. I can't explain it, but it touches upon ideas in such a unique way that I couldn't help but hate it at first, then later come to love it (plus it has an awesome soundtrack). Unforgiven is just... a very good movie. The way Eastwood just stumbles into town underneath a stormy night drunk as hell is hair-raising for me now.
Some others:
Raiders of the Lost Ark. IMO the other Indiana Jones movies are just okay, but Raiders is ****ing beyond awesome. The sense of action and adventure cannot be touched.
The Thing. Best horror film as far as I'm concerned. I mean, seriously it has everything. The monster is creepy and realistic. It looks just like a shape shifting creature would. The isolated location and paranoia stemming from a monster that can disguise itself as the main characters is unbeatable. Oh yeah and it has awesome graphics before Hollywood took a giant cop out with CGI.
Lawrence of Arabia. I had actually bought the DVD a very long time ago and just never got around to watching it namely because I'm really skeptical about old films (I can respect them, but I usually just don't like them to be honest). I saw it in a college class of all places and then watched it again on my own time afterward. Fantastic movie. The scene where they're at the well in the middle of the desert literally creeped me out when I first saw it.
American Psycho. Holy shit I love this movie. Christian Bale's acting is just ridiculous, more so I think there's a lot of unnoticed social and cultural commentary that gets buried against the more obvious murderin'. Plus the dialogue is great, particularly with the... filming scenes. "I'm not trying to get you drunk, but that's some very expensive champagne you're not drinking."