Best Ethics book ever?

There's some interesting discussion (with one or two very distinguished commentators) over at the Leiter reports in response to something Peter Singer said.

Personally, I agree with those who claim that Sidgwick is better than Mill. On the other hand, it's such a lengthy book that I think it'd be hard to know where to begin when using it as only part of a class on ethics generally.

Adam Smith's Theory of the Moral Sentiments, on the other hand, I've never read, and wasn't even aware it was highly regarded. Whether this is some idiosycratic position that I'm in, or if he really is so underrated that many young aspiring philosophers know little of him, I don't know.