American Graffiti
I always find it fascinating that a director responsible for the most famous science fiction story in history could also create the setting, mood and characters that would morph into Happy Days.
Lucas obviously had a feel for the heady days of being a horny teenager in 1950's middle America when rock and roll was just coming to life and things couldn't be better (as much of a feel for the excitement of Saturday movie matinee serials that were his inspiration for Star Wars).
Keeping it simple, the movie tracks the fates of a bunch of friends deciding whether they want to go to college while they pursue girls, drag race and hang out at the diner. It launched several careers and while it didn't break new ground in film making or story telling, it birthed an entertainment movement that would be emulated everywhere from Happy Days to Grease.
If it's not your cup of tea you'll still find it a worthy museum piece even if you don't find it an entertaining movie.