I've been in the market for a new iPod, had a free afternoon recently, and listened to all the Apple audio devices through a pair of Etymotic in-ear monitors at the local Apple store. What I found is that the two far-and-away best sounding devices, and not by a subtle margin, were the iPhone 4S and... the 6G Nano. Perhaps not surprising, because they were the last two to be introduced. I was planning on an iTouch, but it sounded quite flat and gray by comparison. I ended up with a 16G Nano, since I don't need a phone, but the 4S sounded identical.