What is the best book community out there?

We want to know what YOU think!
GoodReads, a favorite of COS, is a wonderful community of readers, but there are so many it's hard to determine which one to invest your time in.
A great deal of Ning sites have sprung up with groups for general readers and then genre specific. Shelfari and even Amazon's community have a great following. But, which one is best?
We'd love to have your opinion!