Oh man that is a challenge Brittnee.  Mostly cuz I don't have a good memory for this kind of stuff.  I'm torn between Forrest Gump and O Brother Where Art Thou?  Also Juno was a strong consideration for me I like that one for Kimya Dawson she's an interesting artist.  Then I was reminded of movies like Almost Famous and High Fidelity which take you back to some of the best stuff in the 70's.  I had to search on google real quick and I came across a movie I'd never heard of 'Singles' but it has what is probably my favorite compilation of songs for a movie, Pearl Jam, Chris Cornell, Smashing Pumpkins (one of there best in Drown), Alice In Chains, Soundgarden, Mother Love Bone, and yes even Jimi Hendrix.  That being said I'm going with my heart on this one because it is treacherous ha, Forrest Gump just plays the perfect songs for each segment it introduces in the movie, and has a nice diversive soundtrack wheras O Brother is more focused on one type of music.  Wish I could have found a good trailer for it, but I did find a good example of what I mean about playing great songs when introducing a new scene arriving in vietnam.