I love all three but for very different reasons.
I love Hairspray because Tracy Turnblad is a character to which I connect with well, and the music and performances are so high energy that you really CANT not love it.
Legally Blonde is fun and high energy and the music is so much fun and the character of Elle Woods has a great journey from ditzy sorority girl to chic law student. She and Tracy have semi-similar journeys in that in the end, they discover who they are as people.
Xanadu is campy, hysterical fun. Its music is catchy and the cast is charming and loveable [Hello, Cheyenne.]
Also they seem like their always having a good time onstage.
"Anybody that goes to the theater, I think we’re all misfits, so we ended up on stage or in the audience.” --- Patti LuPone.