I would say the Zen Micro b/c:
1. good price
2. comes in 4, 5 or 6G capacities
3. comes in several colors
4. FM radio tuner
5. removeable rechargeable batteries
6. comes in a lot of colors
7. good sound quality, it's made by Creative (a "sound" company)
Rio Carbon, nice price, but it does not have a user changeable battery, no radio, and some of it's earlier models had a problem with an improperly ground case, which caused static in some headphones.
OlympusM:Robe: I really don't need to explain this one do I, why would anyone get an mp3 player from a company that makes cameras. Interface is hard to use, and is hard to see in different lighting conditions. No radio, no user replaceable batteries. No.
Dell PocketDJ: Again, why would anyone get a mp3 player from a company that makes computers. This player is supposed to compete with Apple Ipod Mini, yeah right, it can't touch the Mini's slick interface.
If it a choice b/t those player, it's got to be the Creative Zen Micro, best features for the money.