My criteria is that is to have excellent sound quality, good hard drive capacity and optical out, as it will act as part of my hifi set up when plugged in to the DAC.
I am not in the slightest bit interested in iPod for its lack of flexibility, so am looking at other alternatives. Gimmicks are also out, I don't need to be using it as a phone, camera or sat nav and don't need to be playing games of any description.