Pick a favorite movie for each decade.

2000's. The Dark Knight
90's. Terminator 2 (or Princess Mononoke)
80's. Aliens
70's. The Godfather: Part II
60's. You Only Live Twice
30's. City Lights
20's. The Kid
I havn't seen enough off the other decades to make a choice.

00s Saw
90s Star Trek VI
80s Die Hard
70s Star Wars
60s The Great Escape
50s Scrooge
40s Its a Wonderful Life
30s Grand Hotel

Ok, those that know me on Listal know that I'm a huge horror fan, so I'll make two lists:

Favorite HORROR movies by decade:
2010's The decade just started but so far MEAT GRINDER (Thai film)
2000's Rampo Noir
1990's Dead Alive
1980's Re-Animator
1970's Texas Chainsaw Massacre
1960's Night of the Living Dead
1950's Curse of the Demon (aka Night of the Demon)
1940's The Thing from Another World
1930's The Bride of Frankenstein
1920's Nosferatu

Favorite NON-HORROR movies by decade:
2010's I haven't seen any non-horror films yet this year except Book of Eli
(and I wouldn't include that in a "best-of" list)
2000's Ichi the Killer
1990's Naked Lunch
1980's Blue Velvet
1970's A Clockwork Orange
1960's The Wild Bunch
1950's Seven Samurai
1940's Shadow of a Doubt
1930's M
1920's Laugh, Clown, Laugh

50's- On the Waterfront
60's-Hellfighters
70's- A Clockwork Orange
80's- E.T
90's-Fight Club
00's- Blood Diamond

2000s: Lord of the Rings: Return of the King (any of the three will do, and Dark Knight is awesome too)
90's: The Matrix
80's: The Empire Strikes Back
70's: Star Wars
60's: The Good, the Bad, & the Ugly
50's: Vertigo
40's: Casablanca
30's: Bringing up Baby

I haven't seen many pre-80's movies but off the top of my head..

30's - The Wizard Of Oz
40's - It's a Wonderful Life
50's - Some Like It Hot
60's - Mary Poppins
70's - Grease
80's - Labyrinth
90's - La Vita è Bella / The Silence Of The Lambs
00's - Pan's Labyrinth

It wasn't an easy job, but here we go -

20's The Kid
30's Gone with the Wind
40's Gilda
50's Rear Window
60's Breakfast at Tiffany's
70's Sting
80's Stand by me
90's The Usual suspects
00's Amelie

What a fun list to put together! Mine are:

1910's - The Birth of a Nation (1915)
1920's - The Passion of Joan of Arc (1928)
1930's - City Lights (1931)
1940's - Citizen Kane (1941)
1950's - Paths of Glory (1957)
1960's - 2001: A Space Odyssey (1968)
1970's - Apocalypse Now (1979)
1980's - Raging Bull (1980)
1990's - Goodfellas (1990)
2000's - House of Sand and Fog (2003)

This was a really tough list to put together...

2000's - Lost in Translation (runner up- City of God)
1990's - Schindler's List (runner up- Pulp Fiction)
1980's - Star Wars: The Empire Strikes Back (runner up- Raiders of the Lost Ark)
1970's - The Godfather (runner up- Chinatown)
1960's - La Dolce Vita (runner up- Dr. Strangelove)
1950's - Vertigo (runner up- Seven Samurai)
1940's - Casablanca (runner up- The Philadelphia Story)
1930's - The Wizard of Oz (runner up- Mr. Smith Goes to Washington)
1920's - The Cabinet of Dr. Caligari (runner up- The Gold Rush)

Notable films I hated to leave off of the list- Return of the King, Goodfellas, Back to the Future, Alien, 8 1/2, Singin' in the Rain, Citizen Kane, M, and Nosferatu.

00's - Lord of the Rings
90's - Terminator 2
80's - Alien
70's - The Godfather
60's - Lawrence of Arabia
50's - Ben Hur
40's - They Died with their Boots On
30's - Gone with the Wind
20's - Son of the Sheik

10's - The Doll
20's - Pandora's Box
30's - City Lights
40's - Casablanca
50's - The Furies
60's - Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb
70's - Annie Hall
80's - Heathers
90's - The Double Life of Veronique
00's - The Royal Tenenbaums

1900s - A Trip to the Moon
10s - The Birth of a Nation
20s - The General
30s - Wizard of Oz
40s - Citizen Kane
50s - Vertigo
60s - Butch Cassidy and the Sundance Kid
70s - The Godfather
80s - Scarface
90 - Forrest Gump
2000s - Gladiator
10s - Shutter Island

40s - Bambi
50s - Vertigo
60s - The Great Escape
70s - The Deer Hunter
80s - The Elephant Man
90s - Häjyt
00s - This Is England
10s - Kick-Ass

00s - A trip to the moon
10s - A dog's life
20s - Körkarlen (Phantom chariot)
30s - La bête humaine
40s - Il Ladri di biciclette (bicycle thief)
50s - Les diaboliques
60s - Akahige (red beard)
70s - Novecento (1900)
80s - Hotaru no haka (Grave of the fireflies)
90s - Il ladro di bambini (stolen children)
00s - Bin-Jip (3-Iron)
10s - Shutter Island

Ahh The Bicycle Thief, good pick! Can't beat Citizen Kane though.

00`s- Cidade de deus/Revolutionary Road/Love me if you dare (Jeux d'enfants)
90`s- Fight Club/The Shawshank Redemption/Gia/American History X (hard to pick)
80`s- E.T
70`s- Godfather
60's- Breakfast at Tiffany's
50`s - Sweet Smell of Success
40`s - Casablanca
30`s- Gone with the wind

2000's - Children Of Men
90's - East Is East
80's - Blade Runner
70's - Chinatown
60's - Once Upon A Time In The West
50's - The Bridge On The River Kwai
40's - Citizen Kane
30's - The 39 Steps
20's - The Battle Of The Century
10's
1900's - Life Of An American Fireman

00's- Gran Torino
90's- The Shawshank Redemption
80's- Back to the Future
70's- The Godfather
60's- The Good, The Bad and The Ugly
50's- The Bridge on the River Kwai
40's- Casablanca
30's- All Quiet on the Western Front
20's- Metropolis

00's - There Will Be Blood
90's - Fight Club
80's - Fanny and Alexander
70's - Stalker
60's - 2001: Space Odyssey
50's - Rear Window
40's - The Great Dictator
30's - M
20's - Sunrise