It's A Wonderful Life (1946)
When it was originally released this film received mixed reviews. and it actually lost money at the box office despite being nominated for a number of Academy Awards, including Best Picture. It might have disappeared from the American consciousness altogether if not for a clerical error. Failure to renew the copyright of the film allowed it to go into the public domain in 1973. This meant that TV stations could show it for next to nothing. And they did. Boy, did they ever. It became a staple of December programming and re-entered the hearts of viewers as the quintessential holiday film. Not lacking at all in sentimentality (it is a Capra movie, after all), It's A Wonderful Life still boasts some strong performances, and "the phone scene" stands as one of the most titillating moments ever captured on film. It is best viewed within 45 days of Christmas when we are all a little less immune to the overly-saccharine, but it is still a pretty good film.
On the Waterfront (1954)
This is my favorite Brando performance. I've never read anything to confirm this, but I can't help but feel that Stallone based Rocky Balboa in part on Brando's Terry Malloy. The earnest but not particularly bright former boxer is unwittingly drawn into the middle of a conspiracy of corruption, violence and murder in the longshoremen's union. Though there is no final fight with Apollo Creed, Terry Malloy's battle is every bit as inspirational and cheer-inducing as Rocky's - and then some. This movie absolutely belongs in the top 20 of any list of the greatest American films.
The General (1926)
I love this movie. Buster Keaton strikes a perfect balance between comedy and action in this tale of a railroad engineer who uses his engine - the General - to rescue his lady love during the Civil War. Keaton was a master of the visual gag, and this film has many of his best. It's also an endearing love story, even though (or especially because) Keaton's hapless Johnnie Gray is far from the typical leading man.
The Graduate (1967)
This is a funny film. It is a little dark and off-putting at points, though. It has a clever script, and Dustin Hoffman and Anne Bancroft are very good as Benjamin and Mrs. Robinson, though neither of the characters are particularly likable, in my opinion. this is another one of those "iconic" films like Easy Rider that I think isn't quite as amazing as people seem to remember it as being. This is a far superior film to Easy Rider, though, and I do think it belongs on this list, but I also think #17 is a pretty generous ranking.
Sunset Boulevard (1950)
Here's another one from Billy Wilder, which, to me, cements him as one of the world's greatest directors of all time. This tale of a faded Hollywood star's descent into madness is one of his best dramatic films, and Gloria Swanson and William Holden are just perfect casting. I find it amusing that there were many stars of old Hollywood at the time who very deliberately denounced this work of fiction as not being an accurate depiction of older Hollywood stars. Did I mention that this is a work of fiction? We're already pretty high on the list at this point, but I am of the opinion that this film could stand to be ranked even higher.
In 1993, Andrew Lloyd Webber adapted Sunset Boulevard as a musical. It found its way to Broadway in 1994 starring Glenn Close, here performing the show-stopping "With One Look."