Nano - If you do lots of active sport and if you got a Nike+iPod Sport Kit.

Classic - If you must insist to have all your song collection into one device.

Touch - If you want to be the trend to owning such a revolution device.

iPhone - If you really wanted a phone + MP3 + Video being displayed on a nice big screen

I have the old Ipod 30gb but intend to get the Nano 8GB.
Although price differences with the Classic is just RM200++ for a better storage and bigger screen, I have no choice but to get the Nano.

Reason is simply because physical size does matter for me and the light & slim factor suit my outdoor needs (Running & outdoor nature activities).