Middlesex. I'm only about a hundred pages in, but I highly recommend it.
Also, of all the Iain M. Banks I've read so far, Use of Weapons is definitely the best. I sort of dropped halfway through Excession because of school stuff and I haven't picked it up yet, but I have the same problem with it that with everything else from Banks: the early parts are not quite captivating enough, so I have to work through them until the story gets going and it gets a chance to shine.