In approximate order from least to most trashy:
Metropolis (Fritz Lang) - I found it browsing the video store in the 90s, not knowing anything about it. I was amazed and that was just the Giorgio Moroder version. Finding more restored and complete versions later with original score etc. was always great. Of course I had to watch each version multiple times.
Pi - A guy who lived in my dorm 1st year college told me about this. I love the rhythm and repetition. Watching it just feels like a ritual.
Bringing Out the Dead - I'm not sure why I've seen this so many times. But I like it so much. Even people who hate Nicolas Cage because of his crappy movies can enjoy this.
12 Monkeys - I saw this in the theater. Later in film school I saw La Jetee. Like the Twilight Zone, it shows sci-fi concepts in a (mostly) mundane, present-day environment.
The Fifth Element - Bruce Willis really is one of my favorites...before The Whole Nine Yards when he started going downhill.
The Big Lebowski - I'm not sure what it is about this movie. Many people are obsessed with it. There are bars and stores and festivals themed around it. I believe it's because it is the most accessible from the Coen brothers, although I find them always clever and delightful.
Short Circuit (both of them) - The first one was one of my favorite movies shown on the WPIX weekend/late night rotation when I was a kid. I like the sequel even more. Johnny 5 is alternately charming, funny, and cool while Fisher Stevens is consistently hilarious. Although the fact he is really white and not Indian kind of taints his performance for me.
Desperate Living - Not sure if this is the first John Waters movie I saw, but it certainly made the biggest impression. My best friend recorded it on cassette tape and we would listen to it while driving around in his car.
The Toxic Avenger - Troma's weird strategy of making a Saturday morning cartoon out of this very violent and gory superhero story worked, I had to see this as soon as I found out about it.
There are probably more, that I can't think of offhand. But all of these have wonderful cinematography in their own way, as well as great stories.