pick one you(or SWMBO) like, and use amarok to manage your MP3s and your player....
More usful a reply might be, Sony ones have a nice interface but are carp when it comes to placing files on (you can do it by straight drag and drop but don't count on the player being able to see them!)
The easiest ones for use that I have had have been the iRiver, SanDisk and generic ones.