Is there a best year Viper????
They are all good, but they do refine over the years. They seemed to make lots of updates mid-'03 (things like better cooling, etc.). After that, the big changes seemed to be the handwarmers (they made them electronically controlled by thumb buttons, instead of the dial earlier), detonation control stating '04 (some hate this, right?), and on the '04 Viper s they put a ripsaw track (some say was a big improvement there that took too long). On Viper Mountain they only seemed to add detonation control and new handwarmers after 2003.
But a lot of owners kept up on these updates themselves when they found them important. Rather then focus on the model year, i'd check out the machine itself.