First, it was pretty easy to eliminate any athlete involved in team sports. Even at Michael Jordan's level of dominance, a team sport just doesn't allow for a single person to so totally dominate all of their competition.
After that, I looked at the most dominant athlete of the last 20 years, Tiger Woods. As great as Tiger Woods was during his prime, he basically would have had to have his performance at the 2000 US Open in every major for three straight years to be as dominant as Ronda Rousey. Tiger Woods was amazing, and he's not even close to Rousey.
Then I took the logical step of looking at MMA. The most dominant recent champions have been Anderson Silva and Jon Jones. Both guys were dominant, but they had some struggles against opponents. Anderson Silva ripped off 16 straight wins in the UFC, but in there, even he had times of mortality against guys that could put him on his back like Travis Lutter, Dan Henderson, and Chael Sonnen. Jon Jones has never been beaten, but Alexander Gustafsson gave him a lot of trouble, and he usually wins by picking his opponents apart, where Ronda Rousey steamrolls them.
If you want to go old school, you can look at a guy like Royce Gracie who did completely dominate from UFC 1-UFC 4. Until Dan Severn lasted over 15 minutes, nobody made it to the six-minute mark against Royce. Still, Gracie won purely with technique, where Rousey wins with not only technique but by also just being more physical than her opponents.
If I was going for an MMA parallel, the answer would have to be Mark Kerr. Kerr was not only a world-class wrestler who became incredibly adept at submissions, but then he quickly learned how to light people up on the feet and had absolutely brutal ground and pound. He destroyed every one of his first 11 opponents, where only one man was able to last past 3:04. He was the guy picked to rule over Pride. Obviously, that didn't happen, but even if you only count those first 11 fights, the level of competition that Kerr faced was usually not the greatest, where Rousey has been taking out the best of the best since her third fight.
And that brings us up to the most common comparison, Mike Tyson. As weird as this sounds to say about Mike Tyson, he wasn't dominant enough to be compared to Ronda Rousey. Even before he won the title, he got taken to decisions by guys like James Tillis and Mitch Green, who were solid gatekeepers but guys on a level that Tyson should have steamrolled. I will give Tyson credit as he avoided his toughest possible opponent in his prime with Evander Holyfield and Ronda Rousey has avoided her toughest opponent in Cyborg. The nail in the coffin is Tyson's loss to Buster Douglas. Douglas is basically the equivalent to Bethe Correia, a beatable opponent that is meant to showcase the champion in a foreign land. Tyson got knocked out by Douglas, Rousey took less than a minute to annihilate Correia.
After going through all of these greats, it would seem like I have exhausted all of the possibilities. But then I realized the answer was standing in front of me the entire time. I don't know how I could see around his blobby existence, but I managed to do it until now.
Of course, I'm talking about Steven Seagal. He has fought the best in the world and has always come out on top. But that's the thing, he had contemporaries like Bruce Willis, Arnold Schwarzenegger, and Jean-Claude Van Damme who had similar accomplishments, but nobody made it look easier than Seagal. He annihilated his opponents without breaking a sweat and hardly ever having a single mark on his face as a sign that he had been in a battle to the death.
Ronda Rousey is incredibly impressive, and in MMA, she has had to deal with a wide variety of opponents and situations, but it is nothing compared to what Seagal has shown in his fighting career. Just take a look at this highlight video that would even impress Miss Rousey.
Ronda Rousey is great. She is the female Steven Seagal, and there is no greater compliment than that.