Greatest DAP I've ever used: iRiver H300 series with rockbox. Dedicated headphone jack, line in, line out, and built in mic. Not to mention it plays videos, almost every file format with the help of rockbox and the sound quality is great. Thanks to rockbox we finally have a full featured equalizer, as well as gapless playback.
Also, when you finally kill your battery, you can easily upgrade to a 2200mAh battery from an earlier generation iPod to give you 22-28hrs of continuous playback. Not bad for a $20 mod.
Thats got my vote for the best sounding, and best overall, mp3 player.
(the earlier version - H100 series - even has optical SPDIF!)
And regarding the iPods - I have to say that after having two of them before my iRiver, they easily take the cake as far as navigation goes. Other than that though, Apple has their iPod OS WAY too sterile, ie lacks basic things such as a customizeable EQ. And iTunes drove me crazy.