Well... that completely depends on what you want to get out of your MP3 player. If your in it just for music then I am a fan of the iPod shuffle. I love its small form factor and I just throw on my workout songs and take off. Its super light. The downsides-- no real control over your music (other than what's on there as it randomly goes through your play lists). Also if you are interested in adding gadgets such as Nike+ the shuffle does not work with it. Which brings me to the iPod mini- slightly larger form factor. However, its still small and you have all the control over your music that you want. Also, it is compatible with Nike+ if you're so inclined. And then there's the beast-- the iPhone. Though its is by far the biggest and heaviest of the three it has a lot of gadgets (I guess that's what the iPhone is all about). With the built in GPS you can use apps like RK Run (my favorite, as it shows elevation gain and loss in addition to speed) and Map My Run (not as cool). It also has built in Nike+ support if you are so inclined. When using the iPhone I don't use my Nike+ I use RK Run most of the time as I like the elevation gains (and its free-- yes there is a paid version but the free version works well enough for me). Also the iPhone is, as is often neglected, a phone-- so you don't have to carry both. I know most folks don't carry a phone with them on runs. But I like the piece of mind of having a phone with me on longer runs. Ohh.. and its an MP3 player too .  But it is bulky and someday's I just don't want the bulk so I find my mini and take off (or I just leave the tunes at home-- some times the quite time is nice). As you may have guessed I lean towards the Apple line up. They are easy to use and they work well-- both points are key when having a fitness MP3 player. The last thing you want is to figit around with ill designed MP3 players whilst working out.

Anyhow-- thats my two cents (which is worth only about one cent).