This is a question I think about quite a lot because I love musicals, especially seen live on a stage. I am very attached to ones from my childhood like GIGI, MY FAIR LADY, THE KING AND I, WEST SIDE STORY. Some I enjoyed sharing with my kids (A CHORUS LINE)
But some of the more recent ones are pretty swell too.BOOK OF MORMON was a knockout, for instance. And HAMILTON has certainly rocked the musical world. SPRING AWAKENING was great.
There is more than one aspect to consider, of course. Some have weak stories-some have less than perfect scores. And it depends on the production you see.
Here is a recent list from The Guardian.
If I was forced to choose one, I might go with CABARET. What about you?