I don't think the quality determines on the type of MP3 player you have as the MP3 player simply acts as a memory unit without compressing or changing MP3 files of course it also acts as a amplifier to amplify the sound through the headphones but aslong as you get a decent MP3 player rather than an old 5$ chinese one you should be fine, just get an iPod as the quality really depends on the headphones aswell as the MP3 source.
Say for example you have a pair of really good heaphones but your mp3's are from a youtube converter site like
www.tubidy.com then your sound quality will be rubbish as converting from youtube or any video for that really kills the quality, to get good quality MP3's I suggest you rip them from CD's or download full CD rips rather than single tracks.
But also you will need a good pair of headphones, I have listed a few with good reviews. You will need to google the names to find more information:
In-ear headphones:
SoundMAGIC E10 Earphones RRP £34.99
Phonak Audéo PFE 012 Earphones
ClarityOne Earphones << I own a pair of these, they are amazing!
Phonak Audeo PFE 232 Earphones
Over-head headphones:
Audio-Technica ATH-M50 Closed Back Headphones
German Maestro GMP 435 S
HiFiMan HE-500
SoundMAGIC P30
Hope my post helped you.