99-02 is the best looking. Mechanically they are all pretty similar, excluding engine/transmission combinations. I used to own a 99 Chevy (4.8L V8, 4x4) until I traded it in on my GMC. That was the best truck I ever owned, and I regret trading it in everyday.
The 02-06 look is just a little wild to me. It looks like the designers at Chevrolet were trying to put a tougher, more aggressive look on the truck without altering the fundamental construction of the truck. It came out totally wrong to me.