Hm. Well, here's a list of very good movies off the top of my head.

The Princess Bride.
Eternal Sunshine of the Spotless Mind.
Raiders of the Lost Ark.
The Incredibles.
Ghostbusters.
Requiem for a Dream.
Back to the Future.
The Prestige.
The Matrix.
Fight Club.
Jurassic Park.
Alien, then Aliens.
Titanic.
Silence of the Lambs.
Children of Men.

Constantly refreshing the thread to make sure nobody steals my answers, steering clear of Teague's suggestions though I agree with plenty.

Blade Runner
Apocalypse Now
The Good the Bad and the Ugly
A Clockwork Orange (and also 2001: A Space Odyssey...)
In Bruges
Groundhog Day
M*A*S*H
The Man Who Fell to Earth
Kiss Kiss Bang Bang
Chinatown
One Flew Over the Cuckoo's Nest
Sunshine

They're mostly just personal favorites, it's a hard question to consider. And even harder to not just list every movie I like. As for my own missed essential viewing. I never saw any of the Godfather movies until last summer, much to my great shame. And the world at large was correct, two of them are great...

I agree with Teague. Princess Bride is a special case, cause I honestly don't like it that much, but if you've never seen it then watch it, because you can't truly understand about 40% of the internet unless you have.

For foreign movies, I will add:

Seven Samurai / Sanjuro / Yojimbo / Ran / Ikiru / High and Low / Red Beard
(Just about anything Kurosawa ever made, really...)
Hana-Bi / Sonatine / Brother (or Outrage, tho it's not at the same level, really...)
Twilight Samurai / The Hidden Blade / Love And Honor
(Anything Yôji Yamada has done since Twilight Samurai, really...)
13 Assassins
The Chaser
Delicatessen / City Of Lost Children
Let The Right One In (The US version is close-but-no-cigar in my opinion)
Life Is Beautiful / Johnny Steccino (aka Johnny Toothpick)
Hard Boiled
La Strada

For more US / English flicks:

Fargo
Ronin
Heat
Cop Land
Pulp Fiction
Kill Bill
The Terminator / Terminator 2
First Blood
Die Hard
Lethal Weapon

To be continued by another person...

Good thread. That'll help me add stuff to my watchlist (that, and iCheckMovies. God I love this website).

Ya, it's an awesome setup for a thriller, and the movie just drains you emotionally with how much shit it throws at the protagonists. Funnily enough, it kind of represents the end of the 70s wave of successful auteur cinema, and the dawn of the modern summer blockbuster, as it came out the same summer as Star Wars and got completely crushed at the box office.

Also, the fact that is was PG back then is INSANE. The movie would be a hard-R today, there's graphically burned human corpses, people shot in the head, blood, swearing, etc. It's draining to watch as an adult, I can't imagine what it would do to an 8 year old.

I also still haven't seen Wages of Fear, gotta get on that.