Yeah, Metropolis is definitely on my list as well... one of the greatest movies ever, without a doubt. I also like Vertigo a lot. Let's see what else:
The Day the Earth Stood Still
Them
M
Citizen Kane
The Third Man
Touch of Evil <--- my favorite Welles film
Rope <--- A good, slightly lesser known Hitchcock film, filmed in a revolutionary manner
Plan 9 From Outer Space <--- pretty enjoyable, heh
Harvey
EDIT: Oh yeah, Breakfast at Tiffany's _is_ kinda weak, but then again I was never a big Audrey Hepburn fan...
EDIT2: Ah.. I almost forgot, some of Kubrick's earlier films, such as 'Paths of Glory' and 'The Killing' are excellent as well.