I have to put in a vote for Valmouth, as well. Brilliant campy fun, and unexpectedly moving...there is nothing like it. Both recordings are good, though I would opt for the Chichester revival. It has several of the original leading ladies, but caught 25 years later, when they have aged into their roles beautifully. And as much as I love Cleo Laine on the first recording, Bertice Reading is amazing on the newer one. There was also a BBC radio version floating around, which has a generous helping of the wicked dialogue.
My other serious contender for great West End musical is Robert and Elizabeth. Great story, and beautiful lush music, which is served beautifully by the original cast recording (in this case, avoid the Chichester revival recording...there are some fine things on it, but it doesn't outdo the original).