Well, it depends on your needs. If you need large capacity, the Classic would be a good choice. Keeping audio quality in mind though, the iTouch would be a better choice since it uses a Wolfson Chip. This is better than the Cirrus Logic chip found in the Classis and Nano. Dunno why they went this way, probably costs cutting manouvres. Also, to me it seems that the touch has a little better (read cleaner) output stage then the Nano and Classic. I compared it with my dads 3G nano, and it sounds like they coloured the sound in a ''U'' shape. It suits the majority of people, but not me. I demand my player to give me the most flat and most dynamic clean output possible. So for me, it's iTouch all the way.
I might just buy a Classic because of its capacity, and hook it up to my GoVibe through the line-out. I assume that this would bypass the pre-amp circuit, but not the Cirrus Chip I'm afraid.