What is the best-selling iPod to date?

A friend and I were arguing about whether the iPod Touch is the best-selling iPod of all time. I’m finding it hard to find reliable statistical data on iPod sales. Part of the challenge is small changes within models. Wikipedia will tell me the aggregate sales for all models (http://en.wikipedia.org/wiki/File:Ipod_sales_per_quarter.svg) but this doesn’t help me if I want to argue (as I might) that the Nano (or some other unit) is the best-seller. I think because the Touch is so recent that it can’t possibly be the best selling iPod, but I can’t prove it. Can you help? What is the best-selling iPod to date?