Argue if you like (and if you're Skip Bayless, you probably will) but Micheal Phelps is the most dominant athlete I've ever seen (I was born in 1981). Now before I say why, here are the other athletes I think you can make an argument for (please add more if you want):
Barry Bonds (you could do it, but the steroids issue will ruin your argument)
I couldn't think of a football player that stands out as truly this dominant, so I figure there probably isn't one, but again, argue if you want.
I have NEVER seen someone dominate the competition like Michael Phelps. He isn't even racing against other guys who've been training tirelessly for their Olympic moment- he's training against his own personal bests/world records. And let's keep it real- I know NOTHING about swimming. I had to look up distances of various pools I've swam in to see if they were 50 meters in length or not (incidentally, they were not.) But what I do know is the feeling I get when watching Micheal Phelps. I know that each race, he brings so much focus and competitiveness that no one else has a chance. He gets mad when he wins a gold medal, breaks the world record, but doesn't get underneath the time that he thought he could get (getting under 1:52 on the 200m IM is the race I'm referencing, I believe.)
Phelps in the pool reminds me so much of when Michael Jordan would tear people up on the court. There were points where Jordan could obviously not be stopped, and Phelps gives spectators that same feeling. You know he can't be beat. I love watching athletes when all of their talent comes together with hard work and focus.
Keep on rockin' Michael Phelps- you might be a bit goofy, but you're competitive drive is second to none. Finish it off and complete the greatest athletic performance I've ever seen.