The problem I have with Spamalot is that the music is, IMO, meh. And there’s no point in having a musical if the music isn’t great. Here are a few musicals where I think the music really is great. Obviously, YMMV.
South Pacific
West Side Story
My Fair Lady (which I already mentioned, above)
Guys and Dolls (mentioned by several people)
Les Miserables
Miss Saigon
Hamilton
Rent