05-07 (before the body change) is the best. The 05 can be tuned to an 06, but the 06/07 had some stronger internals. I believe the 05 will yield a little better fuel mileage because of the less aggressive tune, but give up a little power. In the end 05-07 is the best Duramax, but good luck finding one for a good price, they are highly coveted. Can probably pick up a newer truck for the same price. Good luck, I wanted an 05-07 Dmax, but could not find what I was looking for and ended up with the Dodge in my sig and have been very happy with it (minus the fuel mileage).