You started the thread with "Greatest" and then went on to talk about most "dominant" which are not always the same.
Sorry, greatest is either Ali or Jordan. How can you be the greatest sports figure ever when 99% of the people in the world cannot pick you out of a line up nor name anything you have done. Greatness is not just what you have done inside your respective sport but what you have accomplished outside of it, using your platform.
Most dominant? Jordan isn't the most dominant NBA player ever. Gretsky, I'll give him that but he wasn't a two way player at all. Still, the Great One is easily the most dominant hockey player of all time but is he more dominant than Phelps? I'm not a swimming guy at all but the guy has 28 medals in the Olympics and the next two Olympians on the list of all time medal winners sit at 18 and 15.
Phelps and Gretsky definitely stack up well and it's arguable that Phelps is easily ahead of Gretksy but if we are talking pure dominance, Alexander Karelin stands head and shoulders above everyone else. He went undefeated for 13 years, and didn't even give up a single point in the last 6 years that he was undefeated. Won Gold at three consecutive Olympics. Again, from 1988 until his silver medal at the 2000 Olympics, he did not lost a single match. Think about that, he did not have a single bad night and this is all against national and international competition. Sorry, 13 year winning streak is the TRUMP card.