A lot of people aren't fans of the '02 because they can be a pain to jet properly...
I have an '02 with the stock carb, vforce reeds, bills pipe. Other than that it has the stock pistons / head. I've never had any problems jetting, but I bought it used :).
I honestly don't have a lot to compare with, because I've never ridden anything else besides my old '90 CR250.
I will say that I wouldn't go any newer than '04. That year they added a throttle position sensor. Other than that from 05-07 its more or less the same bike. Even from 02-07 you won't find any huge changes, just mainly different suspension setups.
If you are really looking for max horsepower, find an 02 and use the extra money to send it to Eric Gorr. I do know that this year responds well to porting and such, you can't really go wrong.