For me certain movies epitomize certain decades here’s my list, let’s see yours.
30s – Wizard of Oz, 1939
40s – It’s a Wonderful Life, 1946
50s – Singin’ in the Rain, 1952
60s – Midnight Cowboy, 1969
70s – Close Encounters of the Third Kind, 1977
80s – Ghost Busters, 1984
90s – Saving Private Ryan, 1998
50’s ----American Grafitti (even though it was released in the 70’s)
60’s----Easy Rider
70’s–Saturday Night Fever
80’s–
90’s–
'30s – Top Hat
'40s –
'50s – Rebel Without a Cause
'60s – Bonnie and Clyde
‘70s – Saturday Night Fever (with a tip of the ol’ fedora to Casey
'80s – Fatal Attraction
'90s –
I need to think about the '40s and the '90s some more.
And set in 1962.
I think that the movie “The Best Years of Our Lives” is much more a 40’s movie than “It’s a Wonderful Life”. Post war America was really not a great place to be. Lots of angst, lots of change, lots of sorrow amid the charging economy and frantic normalcy (sp?). Wonderful Life crashed at the box office for that very reason. America didn’t believe is a Capra kinda world anymore. They didn’t like the one they had, but they preferred the darker vision over the brighter one. My opinion, of course.
Ah, that’s practically the 50s, you know how this decade thing works. 
Which doesn’t appear to be the OP’s criterion. I think he’s talking about movies that sum up the films made in a decade, not the decade itself. Not even the world’s biggest idiot would name Saving Private Ryan for the 90’s on that latter basis.
Mine:
1920s - The Jazz Singer. You ain’t heard nothin’ yet.
1930s - Gone With the Wind. Just because.
1940s - It Happened One Night - mainly because screwball did epitomise the decade, but I hate IAWL. Besides, IHON won the big four at the Oscars.
1950s - The Thing. B-movie, nuclear threat allegory.
1960s - Spartacus. Big, Christ-era epics.
1970s - Dirty Harry. The beginning of the deconstructionist era.
1980s - Fast Times at Ridgemont High. Enter the teen movie, kicking and screaming.
1990s - Independence Day. Huge, flashy event movies that aren’t really very good.
I have no knowledge or opinion of the earlier decades, but I think you guys are crazy with your later picks.
1980s had the following nominees:
Ghost Busters
Fatal Attraction
Fast Times at Ridgemont High
The 80s were not the decade of comedies, but if you felt it was, Porky’s would more properly sum up the decade’s contributions to film. Nor was it the decade of thrillers. My initial thought was Wall Street, but then I remembered that the 80s were all about action.
Both Lethal Weapon and Die Hard sum up the 80s movie experience perfectly. In a pinch, any Arnold movie would do. Predator comes to mind.
Here’s the 90s nominees:
Saving Private Ryan
Independence Day
In particular I want to address the impression that the 90s were all about “Huge, flashy event movies that aren’t really very good.” I completely disagree. The 90s were all about the rise of independent film. As such, Pulp Fiction is the ultimate 90s movie, though you could also make the case for Good Will Hunting or Clerks.
1910’s: Birth of a Nation
1920’s: Greed
1930’s: Gone With The Wind
1940’s: Casablanca
1950’s: The Day The Earth Stood Still
1960’s: Easy Rider
1970’s: Cabaret
?
1890’s - A Christmas Accident
1900’s - A Trap For Santa Claus
1910’s - Intolerance
1920’s - La Passion De Jeanne D’Arc
1930’s - Errrm… All Quiet On The Western Front
1940’s - Errrm… Citizen Kane
1950’s - Errrm… Ah… Gentlemen Prefer Blondes
1960’s - A Bout De Souffle
1970’s - Gates Of Heaven
1980’s - Do The Right Thing
1990’s - Dead Man? Showgirls?
2000’s - Fa Yeung Nin Wa? Elephant?
Yet, none of those were independent films. Except Clerks.
1930’s Gone With The Wind
1940’s It’s A Wonderfull Life
1950’s Rebel Without A cause
1960’s Midnight Cowboy
1970’s Dirty Harry
1980’s Sure Thing
1990’s Chasing Amy
2000’s The Lord Of The Rings
Yeah, I just read the New York Magazine article about Harvey Weinstein (sp?), and learned that Disney bought Miramax in '93, which is far earlier than I thought. So I retract my original position altogether.
Just how the decades appear to me. More than one when there are two separate impressions.
20’s - The Gold Rush
30’s - King Kong, Gone with the Wind (epics)
40’s - The Maltese Falcon (dark thrillers)
50’s - Shane, Creature from the Black Lagoon (westerns and monster films)
60’s - Goldfinger, 2001
70’s - Shaft, Star Wars
80’s - Porky’s, Rambo
90’s - Forrest Gump
10’s: Amarilly of Clothes-Line Alley
20’s: Safety Last!
30’S Twentieth Century
40’s: Casablanca
50’s: The King and I
60’s: Guess Who’s Coming to Dinner?
70’s: Taxi Driver
80’s: Rain Man
90’s: Being John Malkovich
00’s: Spiderman
1930s: His Girl Friday
1940s: Double Indemnity
1950s: The Big Country
1960s: What New Pussycat?
1970s: Shaft
1980s: Witness
1990s: Titanic
Now I’ll look at everyone else’s.
Oops, His Girl Friday was released in 1940, but at least that suggests it might have been shot in 1939.
Screwball typical of 30s or 40s, anyone? I’d still stick with the earlier pre-WWII decade but then I’m no film buff.