In no particular order:
The Warriors
Shaun of the Dead
The Shawshank Redemption
Ondskan (The Evil)
The Iron Giant
The Devil's Backbone
Old Boy
Inglorious Basterds
28 Days Later
Leon (The Professional)
This is really difficult, though. If you asked me tomorrow I am sure I would change some answers. top 25, maybe? I doubt that would even be sufficient. So much to choose from. D:
EDIT: Forgot about Fargo and O Brother. Generally, anything Coen, Raimi, Tarantino, and the early works of Jackson and Romero. Also, although I don't know the names of the movies, some Laurel & Hardy, Buster Keaton and Chaplin is really worth watching, even today.