My favorite was the 3rd Gen iPod Nano, the "fatty".
I only had it for a few months but it was a great iPod. The reason I got rid of it so quickly is because the storage on them was way too weak for me. I need at least 16GB to be happy with my player. A close second is the iPod Classic, easy to navigate and can hold all my music.
I currently use my iPod Classic 160GB at work and in the car, while I use my iPod Nano 6th Gen for working out and carrying around in my suit all the time. My iPad gets some music play time as well but not as often as the other two.