Well... my personal favorite is Hairspray, but I'm not about to say it's perfect (maybe for me, but not for everyone)
So I'd have to go with Showboat, just because it was the first of it's kind and had pretty much every genre possible. Which would appeal to as many people as possible. My 7 year old cousin just liked looking at the big boat in the movie.
I would say West Side Story, but it made me into a wreck for a week, and I'm still a little mad at it....
What is it about Lucy?? I know she's a bitch, and yet I want to be her...