Cowon has a reputation for making the best audio quality MP3 players. Of course the headphone they supply are crap and uncomfortable (at least for me) so it's always best to invest in your own pair of heaphones to get the most out of the Cowon or any MP3 player for that matter.
I bought the Cowon S9 32GB simply for it's audio quality and the fact that FLAC is supported. Being capable of playing videos is a nice plus. The Cowon J3 is an updated version of the S9. The most significant upgrade is a microSD slot so that you can actually expand the total capacity to 64GB if you insert a 32GB micoSD card. Otherwise there are some cosmetic differences.
Cowon also have some players that only plays back audio, but I think the J3/S9 players have the absolute best audio quality of their entire line up.