The greatest cast album of all time is probably... Company (1970) It's brilliant. It captures the show and the performances beautifully. If you can, don't just buy the album, buy the documentary that was made while the cast was recording the album. It too is a treasure.
The first cast recording I ever bought was the Canadian Cast of The Phantom of the Opera, and I have to say it is a far superior recording than the other Phantom cd's. Notice I did not say the performances are necessarily better, but it is a better RECORDING (believe me, i'm a michael and sarah die-hard fan). The sound quality is crystal clear perfection! A sentimental fav. as well:)
I don't care which show wins "best" cast recording, but I absolutely INSIST it must be a Nonesuch release. I am absolutely head over heels for Nonesuch and their beautiful beautiful releases. SO much better than "here's a CD, here's a slip of paper with some songs on it, knock yourselves out".