It really depends on what you plan on doing with the MP3 player, how much you are willing to spend, and how often can you swap out songs (depending on how much space you have.)
I personally have a black 256mb Creative N200. Got it for free. $79.99 at newegg.com while the 512 model is available for $114.99 or something around there.
The Muvo TX is also very nice, both are very similar in size but the TX can be used as a flash drive which connects directly into your usb drive while with the N200 you have to plug it using the cable.
www.newegg.com is where I go for all my MP3 needs with
www.pricegrabber.com for like a ball park price, but I'll pay a little extra for newegg.com because of the service.