This exercise depends upon the notion that, for example, 1971 has more in common with 1979 than it does with 1969. I can’t see that being even a little bit the case.
Ironically, I’m leaning towards 2012: lots of folks on the internet, the President is black, international terrorism is a major issue, global cooperation is the key to looming environmental concerns…
70s: Saturday Night Fever
80s: St. Elmo’s Fire
90s: Swingers
00s: Up in the Air
Another way to express it is, “What’s the decadest movie of the decade?” Like, what’s the 80sest movie of the 80s, which is clearly “The Breakfast Club”.
I don’t see how any 80s movie comes even close to out 80sing Top Gun.
Lots of interesting choices.
I’ll add WarGames as an 80’s movie to the bone. Had the Cold War/Nuclear Holocaust threat hanging over it, the hacker kid in his room (the 80’s saw the first home PC boom), plus even had a few scenes in an arcade. All of it adds to a movie that couldn’t have been set in a decade other than the 80’s.
I agree with many of the choices, but I’ll try to post some that haven’t been listed to expand the pool a bit:
'30s: It’s a Gift
'40s: The Big Sleep
'50s: North by Northwest
'60s: The Hustler
'70s: The French Connection
'80s: Flashdance (it’s really hard to think of anything other than a John Hughes film)
'90s: Glengarry Glen Ross
'00s: Eternal Sunshine of the Spotless Mind
'40s: It’s a Wonderful Life.
'70s: Gotta agree with “Saturday Night Fever.”
'80s (My decade): “The Breakfast Club.” With a spotlight on the song encapsulating the '80s, “(Don’t You) Forget About Me.”
'90s: I’d go with “American Beauty,” too. Or maybe “Clueless.”
'00s: “Wall-e.” For a movie not set in the '00s, it sure did sum up what everyone was talking about.
1960’s - Yellow Submarine.
I mean, seriously: does any other movie capture the period so completely? Other movies may show the 60’s, but this one actually gets the viewer to *feel *what it was like to drop acid while listening to Beatles records.
For the 70s, I’d go with Apocalypse Now. Made in 1979, everything about it screams “70s” - from the use of the Doors’ music, through the Vietnam War setting, through the general messed-upness of everyone on set. 
But especially the feel of it - that everything is going to hell in a handbasket.
For the early 80s, I think I’d go with Valley Girl. For the late 80s, it’s a toss-up between any John Hughes movie and Wall Street. The first group captures the teenage angsty part, and the second one captures the “fuck you, I’ve got mine” part.
Why not Logan’s Run from the 1970s? Not only do all the “futuristic” people look like they’re from the 1970s, but you have a hedonistic society that produces nothing of substance. Sure sounds like the disco era to me. (Not that a lot of good stuff didn’t come out of the 70s. Hell, I even like disco.)
The animated Transformers, perhaps? Course, I was a kid in the 80’s, so my memories of them are a kid’s memories.
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And the 2000’s Transformers embilizes what art in the 00’s was all about, strip-mining existing pop culture. Except that’s mostly what they did in the 90’s, too, so I guess that doesn’t work.
Logan’s Run for the 70s isn’t bad. I still prefer Apocalypse Now (or if one must go for science fiction, Zardoz for that “bad 70s drugs” vibe )
70s - for me, almost everything about Breaking Away screams 70s.
80s - Pretty in Pink. (Although I watched Echo Park a while back, and while it is not a very good movie, it really has a strong 80s vibe going on. Made me kind of nostalgic. Earth Girls are Easy also deserves a mention for its overwhelming 80s-ness.
90s - Singles for early 90s. The Matrix for late 90s.
00s - As a metaphor, The Dark Knight, I guess. War on terror and all that.
For the 90s, Fight Club. 80s, as someone mentioned earlier, St Elmo’s Fire. 50s, The Wild One, although I wasn’t there, but it’s the one that feels the most of it’s time for me.
60s is probably the hardest one, due to the massive change in films in that decade, probably more than in any other since the 30s. If I had to pick one, probably The Good, The Bad and The Ugly.
70s, hmm, Five Easy Pieces is good for the transition from 60s to 70s, Dirty Harry has the bad hair and the violence, and The Last Waltz, probably accidentally, the excess. Taxi Driver is probably the closest to combining all of them.
00s, this is difficult. I’ve probably seen too many recent films to just pick one, but the earlier suggestion of The Dark Knight is not a bad one.
50s Blackboard Jungle
OK, now I’m going with Taxi Driver for the 70s. Taxi Driver is oozing the 70s from every orifice.
A movie says more about the time it was made than the time it was set.