I too have pondered this question and have come to the conclusion that it isn't so much model years with the S80 but drivetrains. Certainly 2.9s got better over the years but the 2.5Ts were always good. They started in 2004 and ran through the end in 06. By 06 they had dropped the 2.9 altogether so everything was 2.5T. The 05/06 2.5Ts had a better (and indeed very good) EPA rating (21/30) than the 04s (18/28). The 05s still had 16 inch wheels standard then the 06s got 17 inch. To me this points to an 05 being the best year, but a matter of personal choice on wheels. These cars aren't cheap, yet, mid to high teens, but almost certain to go down as the best S80s. They have the engine and transmission that served the S60s so well. Just a whole lot more car than a Nissan in my view.