Hamilton. A work of genius. Will not be surpassed. Very American though.
Book of Mormon. Very funny.
A Funny Thing Happened On The Way To The Forum. Also very funny.
Into The Woods. I like the music, some of it.
Mamma Mia. ABBA wrote a nice tune or two, a comment which does not include the title tune.
The Music Man. Reminds me of my old man, who loved it. It's okay.
Every other musical I've seen has been cringe-inducing -- especially Cats which is dreadful on every level and A Chorus Line which is fucking abysmal. But I haven't seen that many (Les Mis, e.g.) because I don't especially care for the art form.
And I really believe the lyrics for Phantom of the Opera were actually written by a nine-year old.
Edit: I would probably like The Producers but I haven't seen it on stage, just the movie. And Spamalot doesn't really count.