Wow! - What a lot of great songs have been mentioned here! There are a couple of obscure Harold Arlen musicals that have wonderful music - "Jamaica" & "House of Flowers" - hard to find, but if you can get your hands on the recordings, do it. The original cast of Jamaica included Lena Horne, Ricardo Montleban (yes, the rich, Corinthian leather guy, lol), Adelaide Hall, Ossie Davis, & Alvin Ailey. House of Flowers had Pearl Bailey, Ray Walston, Juanita Hall (who, I believe, was the original Bloody Mary in South Pacific), Alvin Ailey, and a very young Diahann Carroll. Both are great for women singers to study phrasing & inflection.
Of the more well-known musicals, at the top I'd have to put La Mancha - fine songs, both musically & as character-defining vehicles. Richard Kiley & Joan Diener were terrific.