It's pretty simple, the best in sound quality are the Hifiman801>602>Sflo2 (In this order) because they are specialized in sound quality, the rest are far behind unamped. These of course have their issues in UI and size, but for sound there's no comparison.
Regarding mainstream players (ie Apple/Sony/Sansa/Cowon) unamped, there's not really one that's vastly superior to the others when you pick out the best ones. No matter how much people praise a mainstream player in the end all the top ones are very close and each has their own sonic flaws. In this case, players like the Fuze, J3. Sony X and Ipod Touch etc. are pretty much close in terms of SQ there's not one that's vastly superior to the other, just a different taste in presentation. As a note, I've tried them all (Touch2g/Sony X/J3/Fuze/Clip+), so I'm not just basing things off of reading.
But then if you want to use a mainstream player with an amp, the best choices are the Ipods and the Sansa Fuze due to the Line Out function.
I think this pretty much sums things up at the moment.
edit: The C30/Gumstick/Rocco are players to mention and fwir (no experience with them) they are somewhere in between the Audiophile players and the mainstream ones. Beating out players like the Fuze but not quite getting there with the Sflo2.
Edited by Inks - 11/1/10 at 1:09pm