depends on what you need. Do you like FM radio? Do you know people with Zunes? If so, do they share your taste in music? If you answer yes to all of those, then a zune is a great choice.
Do you want a lot of cool accessories for you mp3 player? Do you want a wide array of purchasable games, movies and videos? Do you want style? If so, get the ipod.
Do you want a huge screen? Do you want great video quality? Would you mind spending a little more for a DVR function? If so, get the archos.