There have been soo many BIG years in modern snowmobiling, however the term "modern" is to be interpreted. One could look at the advancements in suspensions both front & rear including the slide rail. Plastic skis, the introduction of the smart shocks in 2000 (which were an expensive pita). IFS to AWS & long travel rear suspensions. EFI, push button reverse, diamond drive vs chain case. 4 stroke engines, high output twin two strokes, turbos. I would have to agree with the rider forward design as a huge improvement as well, but, all of the advancements made in sleds over the years, it is diffiicult for me to say which was the "biggest" because all of them are huge changes & sleds wouldn't be what they are today without any of them, including a trail system.