A completely arbitrary, mainstream, and unoriginal list without useful commentary, information, or even links to IMDB.

8 1/2
Glorious final scene is so enriching it still blows me away. The most human of movies.
The Maltese Falcon
"You always have a very smooth explanation ready." "What do you want me to do? Learn to stutter?"
Broadcast News
Beyond category portrayal by Albert Brooks. Holly Hunter has never been so perfect.
Star Wars
Still brilliant. The opening shot with the two space ships coming into view from above and gliding into the screen is masterful. The entire sequence following the two robots' adventures until Luke is introduced remains charming to me.
"Round up the usual suspects!"
A movie of perfect structure. Not one shot wasted.
Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb
The darkest possible comedy. "I do not avoid women ... but I do deny them my essence."
North by Northwest
Cary Grant! Eva Marie Saint! James Mason! Thomas Jefferson and the rest of the Mount Rushmore gang! Hamlet! Easily my favorite Hitchcock. One of the greatest movie scores I know.
Five Easy Pieces
"I want you to hold it between your knees."
Raising Arizona
Still my favorite of the Coen brothers movies. Holly Hunter has never been so perfect.
To Catch a Thief
Cary Grant! Grace Kelly! What is the deal with Grace Kelly anyway? Was she beamed down to earth by aliens?
Shows an America I recognize not from having seen it on TV, but from having lived here my entire life. Unforgivable scene with Frances McDormand meeting her Asian high school classmate is horrific blemish.
High Noon
Unity of action. Unity of time. Unity of place. Unity of Grace Kelly.
Groundhog Day
I love romantic comedies. Why aren't there more on this list? If Andie MacDowell can't act, why has she been in so many movies that are so pleasurable to watch? (In addition to this, Four Weddings and a Funeral and Sex, Lies, and Videotape come to mind.)
The Unforgiven
Wonderfully slow-paced Western without answers.
The only documentary on this list so far. Filled to bursting with goodness. I suppose it should be Helvetica.