TOD:
unquestionably, without hesitation: FOLLIES
That's high on my list, too: never has any show used its title and the title's connotations so fully and accomplished so much
I have to add, as well,
THE FANTASTICKS (commedia dell'arte, presentational, non-realistic)
COMPANY (without this one there would have been no FOLLIES)
BIRDS OF PARADISE (Chekhov's THE SEA GULL but only if you know THE SEA GULL)
A LITTLE NIGHT MUSIC (the closest to a Chekhovian musical not based on Chekhov)
PROMENADE (Bernstein's CANDIDE moves to Ionesco territory)
LITTLE SHOP OF HORRORS (take a bad movie, push the Faust references and make a masterpiece)
MUCH ADO ABOUT NOTHING (NY Shakespeare Festival, 1972) (the best production of Shakespeare's comedy I've ever seen)