Jimmy is a bit under-appreciated. He had a solid five years in a row as the year-end #1 (although probably didn't deserve 1977 and 1978, with Vilas having a better year in 1977 and Borg having become the best player in the sport by 1978). By 1978 he was surpassed by Borg and then McEnroe and their "sexy" rivalry. By the time the 80s came about, Jimmy was the old man (30-something) on tour. Yet unlike a player of similar longevity, Andre Agassi, Jimmy was the best player on tour for several years, with his own reign of greatness, and was incredibly consistent. Andre had 1999 but that's about it in terms of him being the best guy on tour.
Anyhow, I think Jimmy was the best player from 1974-77, then was one of the five or so best for another ten years...a remarkable run.