Someone (Schlamp?) brought up the possibility of Alexander Karelin as a candidate for athletic dominance. Yes.
Karelin went undefeated from 1987-2000. I'll give him 13 consecutive years of dominance, which puts him one above Mariano Rivera. As The Experiment, Karelin may also have the best nickname. We'll see how long Rivera is on top, it seems to me he's slipping a little. Just a little. He can't pitch forever (right?), and Papelbon or K-Rod may become the guy baseball people most trust to get out of an inning if their lives depended on it. But my sense is that Rivera is still the man, until further notice.
I also thought of Steve Redgrave. Who? Just a guy who won gold medals in rowing at (ahem) five different Olympics. But he wasn't always the best. He came in third in 1990 in a race. Plus, crew is such a team sport. Except for singles, you're always part of a team, and everyone is doing the same work. It's tough to say that anyone is the "best". Redgrave may be the greatest rower in history, but it's difficult to prove that he was the best rower in the world at any one point.