Despite popular opinion on here, iPods are still 'top-dog'. They now have the best price/capacity ratio as well. (most 20Gb players are £200, £210 will get you a 30Gb iPod).
The Zens are popular on here, and the iRivers are the 'geeks' choice due to their capibilities.
If you just want a music player, get a iPod. Cant beat them.
If you want to use it as a HDD as well, get an iRiver.
If you want a radio as well, get a iRiver or Zen.
Think carefully about what you want, and choose from that.
Think about what your actually going to use rarther than what would be nice/a temporary gimmick.