what are the must see classics?

Everybody's got favourite movies... but which are the ones that you think everybody should see (and like damnit!)?  I mean besides Star Wars because that's just obvious.

I think Robocop 1, 2, and 3 are on that list for me.  Robocop 1 is great: it's a modern Frankenstein story set in a corporate dominated dystopia.  For those who've not seen them, the Robocop films are surprising anti-corporate and cynical about the direction our society is taking.  They're not just action films.  2 and 3 were written by Frank Miller (Sin City, 300, Dark Knight Returns).  Robocop 3 is a little cheese, but get this: Robocop quits the police, joins up with a bunch of squatters and helps them fight the police and robotic ninjas.  He even gets a jet pack!

Taxi Driver is also on the list for me. De Niro's character totally freaks me out, but I can't help but sympathize with him.  He wants to do good, and he doesn't want to be alone.  But he's so confused and not sure how to do it, and still manages to be a hero in the end by developing a strange fixation on a 13 year-old prostitute.

Unforgiven.  This is Clint Eastwood's masterpiece if you ask me.  Is the old man that Eastwood plays the same high plains drifter from his early westerns?  Notable especially for this dialogue (Eastwood is Will Munny, Little Bill is the corrupt sherrif):

Will Munny: Who's the fellow owns this shithole?
[pause]
Will Munny: You, fat man. Speak up.
Skinny Dubois: Uh, I... I own this establishment. I bought the place from Greeley for a thousand dollars.
[Will levels the shotgun, and speaks to someone standing behind Skinny]
Will Munny: You better clear outta there.
Man: Yes, sir.
[scampers out of the way]
Little Bill Daggett: Just hold it right there. Hold it...!
[Will shoots Skinny. Screaming, the women scatter upstairs]
Little Bill Daggett: Well, sir, you are a cowardly son of a bitch! You just shot an unarmed man!
Will Munny: Well, he should have armed himself if he's going to decorate his saloon with my friend.

Requiem for a Dream..  Watching this movie leaves a big hole in my stomach, every single time.  Watching the characters destroy themselves with their vices it's easy to see how it could happen to any of us.  Aronofsky's first feature (Pi) is also really good.  I'm still waiting to see The Fountain.

What about everybody else?  Which films are part of the canon that should be watched by all?

Jeremy