Modded, probably a 4g iPod with the Red Wine Audio line out mod, to an amp and then some nice expensive 'phones.
Unmodified, I like my Zen Micro, the iPod shuffle is supposed to sound great, as is the 5g iPod. The Rio Karma was once loved, but since Rio is dead it's hard to find one now. The Zen Vision: M is supposed to sound great also.
There's not really a 'Best' mp3 player, they all have strengths and weaknesses.
Everybody has a different opinion on what's the best sound quality portable player that it's hard to pin point a overall winner. It all depends on what you can afford and what sounds best to you. What may sound good to others does may not necessarily sound good to you or vice versa.
The best potable player depends on what pot you intend to use to put the player in , a crock pot? a stew pot? different pots for different players I always say.....
oh ! or is it a typo?? The best pooltable player is minnesotta fats.
I went through several players,Creative Zen micro,5G ipod and now I have a 4G 60GB Photo iPod that has been modified by www.redwineaudio.com with the Redwine iMod. With my Redwine mod and a Ray Samuels hornet amp and Sennheiser PX-100 headphones for walking and at the gym and Grado RS-1's at home I have achieved audio Nirvana! My Redwwine mod has 225 hours on it and the Blackgates have burned in nicely. Also I'm using Apple iTunes Lossless.
I would think it would be hard to beat this combo for overall sound.
I think Macmall or PC mall has good prices on 4G ipods.