The one I've watched the most and can never pass up is West Side Story. It's a great adaptation of an adaptation, but it's the dance numbers that draw me in the most -- "America," "Cool," "Dance at the Gym" and the opening number are all pretty spectacular. Rita Moreno is a queen.
Speaking of dance numbers, when I think of what I love most about movie musicals, big dance numbers are probably at the top of the list. The Stormy Weather finale with the Nicholas Brothers is something else -- I dare you to watch it without your jaw dropping to the floor. The fantasy ballets in both Singin' in
