How many people are the best in the world at what they do? Usually, it's hard to even know who's the best in the world at anything, much less find the person who's the best at it.
Now consider, how many people have been the best in the world at their chosen profession for twelve consecutive years? Mariano Rivera's had some competition lately: Papelbon's been great the last couple of years, and plenty of other closers have had individually better years. But since he became a closer in 1997, pretty much everyone in baseball would choose him to pitch the bottom of the ninth with a one run lead.
Has any athlete in history ever been the best at what he did for twelve consecutive years?
My first thought was Edwin Moses. Undefeated from 1977-1987, he won Olympic Gold in 1976. He was at the top from 1976-1987, but by 1988, he "only" got the bronze in Seoul. He didn't last 12 years.
You could make a case for Rickey Henderson. He was the best leadoff hitter in baseball starting about 1980, but I don't think he was necessarily the best for 12 years. Henderson was hurt in 1987, and Vince Coleman was probably better. Tim Raines was undoubtedly better in 1986. Henderson was the best for a long time, but not consecutively.
What about Bill Russell? If one assumes he was the best center in basketball starting in the 1957-58 season, when he was the league MVP, I'm not sure he lasts 12 years. I don't follow basketball closely enough to know, but according to wikipedia, Russell was falling off by the 67-68 season. Maybe Russell gets ten or eleven years, but that's not as impressive as Rivera
Am I missing any other plausible candidates?
(As a sidenote, Edwin Moses was a double major in college in physics and industrial engineering? So he's like Ed Cho, but faster and with an additional physics major.)