I have 3 currently.
3g 15gb
5g (RevA) 30gb
nano (RebB) 2gb
3g 15gb
My first iPod, bought it new. It is touch sensitive for all buttons and actions, meaning you don't have to press on it to use it. Works great in my pocket , and the backlight for the buttons is a nice feature to have in the car when I have it set for the Backlight to stay on constantly
5g (RevA) 30gb
Use this primairily for videos for both iTunes and iPod playable videos, along with avi, mov, mpeg movies that I watch on my computer. It is usually full, and I tend to rotate the ripped and compressed DVD movies (all of which I own) in and out of it.
nano (RebB) 2gb
Not sure yet, I just bought it today, but my plans are to use this iPod for Motocross practice, training in the gym, and for times I don't want my larger 3g in my pocket.