One city that I've seen get a lot of praise is Streets of Silver. I don't have it but it's supposed to be a Venice-y Renaissance era city. It is very detailed from what I've read.
Of cities that have been detailed in past editions, besides Greyhawk, Waterdeep stands out of course. Supposedly, a Waterdeep sourcebook is on the way sometime in 2005, but I havent heard anything on that for a while.
Oh, and I recommend Geanavue: Stones of Peace by Ed Greenwood, for the Kalamar setting. They've also put out an accompanying book Loona: Port of Intrigue, but I haven't got it yet.