Must see movies

The Machine Girl Because you really should experience how much WTF can go into a single movie
Birdman for surrealist drama
Pitch Black

and the stuff that other people have already been mentioning

Waking Life
Bug

Blade Runner
Inside Out
The Godfather
The Lion King
Alien 1-3
The Land Before Time
Donnie Darko
Disney's Aladdin
The Shawshank Redemption
Wall-E
The Fountain
Jurassic Park
Pi
23 (not The Number 23, though that's a good movie as well)

I just recently watched the Blade Trilogy and it was pretty awesome :D

I'm a big fan of The Matrix and it was kinda like that but with vampires and a black Neo :P

My favorites:

• Léon
• The hundred-year-old man who climbed out the window and disappeared

<3

Depends on what you movies you think are "good".
I can suggest several, but no sure you'll like these:
Solaris http://www.imdb.com/title/tt0069293/
Stalker http://www.imdb.com/title/tt0079944/
Blue Velvet http://www.imdb.com/title/tt0090756/
Mulholland Drive http://www.imdb.com/title/tt0166924/

I'm trying to add something different, because we all know everybody was going to say the same films. Lets try a different angle of good/great movies.

Kikujiro (Kikujirô no natsu). A sad kid has, due to bad luck I guess :P, a tough man as a babysitter. Both start a journey to see the kid's mother. 9/10

Seppuku (sometimes called harakiri). Not the remake, the 1962 classic. The 'casablanca' of the asian filmography. Truly a masterpiece, even if you can see the cheap wigs they used. A masterpiece. A story abut the true 'honour' of the samurais.

Pusher. Nicolas Winding Refn 'opera prima' and the film that settles his bases or his pillars... Every movie from him its the same and everyone its Pusher (probably the next one could be different because the star is female)

Das Weisse Band. Tough. Made me feel sick for a couple of days. I felt dizzy at the end, I confess I even went to the bathroom, thinking I was going to puke. This is one of those films you only see once. Never again. Masterpiece.

Hotaru no Haka (Grave of the Fireflies). Same as before, you will feel awful. Take tissues. I don't have the courage to see this again. Same as before, only once, but you must.

The act of killing. Again, same as before. Its a documentary. I still doesnt believe it. For me its all fiction, its a movie, its not real.

Le trou. Ha! This french film normally is not mentioned when people talk about 'the nouvelle vague'. Forget about the bad sounds, the desincronitation of them... Its so good, so so so good.

And a lot more, but I'm going to add finally a swedish and a spanish one:

Saraband: I think its the last one from Ingmar Begman and I think its a TV movie. Of course thats not a bad thing. Its way better than the 90% of the movies made this year and the last one, he. Also it shares characters from another one from 30 years ago but its not necesary to see it.

No habrá paz para los malvados. Despite his tricky script and that maybe you'll need a second vision to fully undestand what the fuck is going on with the investigations I think its a good thriller. If you don't like how it sounds try 'La caja 507' from the same director and with the same actor.

The Last Emperor (1987)
Lawrence of Arabia (1962)
Terminator 2 (1991)
Scarface (1983)
Spirited Away (2004)

There are a lot of great movies already on this thread :P

These are a little bit wierder, but fantastic movies:
Moonrise Kingdom
Ruby Sparks

I'd say What we do in the shadows ...

World of Tomorrow
Whiplash
The Shining
Reservoir Dogs
2001: A Space Odyssey
Grave of the Fireflies

Only movies I'd give a 10/10 so far.

My personal favourites (in no particular order)

There Will Be Blood
Eternal Sunshine of the Spotless Mind
A Bittersweet Life
Inception
Kill Bill
Perfect Blue
Big Lebowski
Seven
Princess Mononoke
Moon
Leon
Oldboy
The Dark Knight trilogy
Drive
Interstellar
Monty Python's Holy Grail
Pulp Fiction
Pan's Labyrinth
The Shining

...that's all that comes to mind right now.

• Blue Velvet
• Lost Highway
• Jacob's Ladder
• Eraserhead
• Any Sergio Leone movie
• El topo
• No country for old man
• The deer hunter
• True Romance
• Reservoir dogs
• My friends
• 8½
• Salò, or the 120 Days of Sodom
• Citizen Kane
• Blade Runner

The Escape ( English )

Wu Xia ( Chinese but its good Martial Arts Movie) its Not Like Kung Fu Hustle pretty Scientific Martial Arts Movie

Red Cliff 1 + 2 ( its Chinese but now available in English )

Fight club
V for vendetta
Seven
Butterfly effect
The boondock saints
Ex machina
(written down without thinking much about it. there are probably more I'd recommend :)

Troy
Training Day
The Devil's Advocate
Scarface
The Patriot
Lock Stock & Two Smoking Barrels
Snatch

Blade Runner - as others have mentioned
https://www.youtube.com/watch?v=yWPyRSURYFQ

Brazil - this movie predicted today 30 years ago
https://www.youtube.com/watch?v=nSQ5EsbT4cE

Like Someone in Love - Abbas Kiarostami
Eraserhead - David Lynch
One Flew Over the Cuckoo's Nest - Miloš Forman
There Will Be Blood - Paul Thomas Anderson
The Thing - John Carpenter
Иди и смотри - Elem Klimov
Millennium Actress - Satoshi Kon
Fallen Angels - Wong Kar-wai
The Tale of the Princess Kaguya - Isao Takahata
Love Exposure - Sion Sono
Tabu - Miguel Gomes
Caligula - Tinto Brass

Just listing a few:

Good Will Hunting
A Beautiful Mind
My Neighbor Totoro
Princess Mononoke
Fantasia
The Thief and The Cobbler Recobbled Cut (NOT the miramax version)
Crouching Tiger Hidden Dragon (the original)
House of Flying Daggers
Pan's Labyrinth
The Orphanage
Indie Game The Movie
Kung Fury

redada asesina (The raid)
Kung Fu Panda 1 / 2 / 3
venganza 1 / 2 / 3 (Taken 1 / 2 / 3)
Transformers 1 / 2 / 3
Sin piedad (The Jack Bull)
battleships
Toy Story 1 / 2 / 3
Resacon 1 / 2 (The hangover 1 / 2 )
Fuga de cerebros 1 / 2 (spanish lenguage only i think)

and a lot more of good movies there's thousands of good movies to must see.

I won't tell you about any famous ones, because everyone knows about the famous ones. xD I'll tell you a few awesome movies of medium-zero fame:
Die Welle, Das Experiment, El Laberinto del Fauno, Dog Pound, Papillon, It's Kind of a Funny Story, La Vita e Bella, The Perks of Being a Wallflower, World's Greatest Dad, Intouchables (ok, that's a bit famous), Amici miei, and any old movies with Charlie Chaplin, Laurel and Hardy, Bud Spencer and Terence Hill, etc.
I can suggest you a lot of awesome movies, but start by checking these out first. ;P
Some famous ones that everyone should see for at least once in their life are obviously movies like The Lord of the Rings, V for Vendetta, American History X, Fight Club, etc. ;P

Watch Four Lions.

the brazilian "tropa de elite" elite squad one and two + "cidade de deus" city of god

Several of my very favorites have already been mentioned: 'Children of Men', '28 Days Later', 'Moonrise Kingdom'. But here are a few I haven't seen mentioned yet:

• 'Open Range' - probably my favorite western, it has great dialogue and unusually down-to-earth gunfighting
• 'Big Country' - An older western. Has both Gregory Peck and Charleton Heston in staring roles, and sort of subverts the bravado typical of older westerns.
• 'Band of Brothers' - A bit of a cheat, as it's actually an 11 hour mini-series. But, if you overlook that, this is hands down the best war movie ever made.
• 'Falling Down' - A drama that starts off looking like a funny revenge fantasy, and then ends up being something much more thoughtful.
• 'Constantine' - This movie is better than it ever got credit for. I concede it's a favorite rather than a true great. But it's amazingly shot, and very visually memorable.