I would say 1991, the year Lee Haney broke Arnold Schwarzenegger's record. He went out in style, truly saving his best for last, as many would agree that he was in his best shape ever that year.
As The "Awesome One" stated on his interview for ESPN, when referring to himself and his condition at that show, "It would have taken something monstrous to be mass, symmetry, and definition.....and Arnold has retired!!!"
Honorable mention - 1998: The emotion in the arena, when Ronnie Coleman realized that he won it all, collapsing on the floor with tears in his eyes and needing several bodybuilders to help him up.