The thing I like about lists like this—and the enthusiasts like me who devour them—is the way they serve as starting points for debate. Fer instance, I think Unforgiven is overrated, although I have a soft spot for Pale Rider, which most critics seem to feel was second-rate Eastwood.
Moises Kaufman has written 33 Variations, a play that explores Beethoven's obsession with Diabelli's inane little waltz. Sounds worth seeing—or you could buy this.
I don't know about you, but I got a little bit bored with the wall-to-wall coverage of the 40th anniversary of Sgt. Pepper's last week. So why am I linking to Jody Rosen's article on the subject? Because it actually has something to say and says it well.
Okay, no list of 50 could be perfect, but this one's pretty good. I could probably come up with a list of 50 that also changed music without any overlap, but that's one of the pleasures of the list game.