I've mentioned a few times before that I don't particularly like musicals, but I love the ones I do. My favorite is probably Singin' in the Rain. I've had the opportunity to see it in the theaters with a crowd a couple times in the past few years. It's always a great experience, and watching certain songs is just pure joy.
In my opinion, the absolute best part of the movie is Make 'Em Laugh. If this scene were filmed today, it would be edited over and over again to make it work (unless it's Joseph Gordon Levitt). But the original? The talent oozes off of the film cells.