I've said this many times over, when thinking about the best musicals of all-time, you have to think of many things other than if you love that belter song in it. Or that the cast is fabulous. It should be measured on the quality of the work, the contribution it made to musical theatre and MAYBE the popularity.
Here's my top ten (in alphabetical order)
A Chorus Line
Cabaret
Company
Follies
Gypsy
Little Johnny Jones
Oklahoma!
Showboat
Sweeney Todd
West Side Story
It's no surprise that 5 of the 10 shows are by Stephen Sondheim, and 3 involved Michael Bennett.
But in my opinion, any "top 10 ever" list that doesn't include Chorus Line, Oklahoma, Gypsy, West Side, Showboat, and AT LEAST one full-Sondheim show, is not a top 10 list.
Eeeeeeyyyyyyyyaaaaaaaannnnnddddd aaaaaaaiiiiiiiiyyyyyyaaaaaammmmmmmm teeeeeeeelllllliiiiiinnngg yyyyooooooouuuuuuuwwwaaaahh...