Definitely the first two Nanos. From what I gather the original black/white one is the best sounding, but it is limited at 4GB and is notorious for the exploding battery issue, though whether it is really widespread or not is unclear. Up to you on whether to take that risk. With 20 year old batteries it's dubious anyway.
3rd gen is worse, but not terrible at all. I am not familiar with the 7th gen but I can assume it sounds better than the 4th-6th gens given the general improved trend of Apple's DACs in that era.