If you don't use EQ, then you can use any ol' average mp3 player. No mp3 player I have heard really had a strong advantage over another that I have heard on flat settings. However, if you like to EQ, the Cowon is a must have. After learning its settings, I haven't heard much else that could compare. Maybe a Samsung P3.
Itunes is also like any ol' media player if you don't use EQ. It's included EQ is horrid though. I've since switched to foobar only for VST bridging of EQ VSTs capability. If I didn't EQ, it wouldn't matter so much, but I believe having an excellent EQ is a necessity when it comes to headphones.