You skip a day, then a day is three, and six, and...
Didn't bother with the Mayweather-Pacquiao results because nothing was surprising, except Mayweather throwing more punches. I suppose it could be argued that a healthy Pacquiao, with an output in the 700-800 range, could have made the fight more interesting considering he didn't get absolutely crushed on the scorecards as it was.
Ronda Rousey is all over the goddamned place. She's the most dominant athlete alive (and, to give an idea of how uninteresting SI is at this point, the second link- to Business Insider [!]- gives a pretty reasonable top fifty list, while SI just glows about her, like they glow about anyone they're fans of at the moment). She released her autobiography (along with a nifty excerpt). She continues to have her opinion asked on everything.
But let's get back to the dominance question. Subjective lists equal inevitably arguing, so it's not a matter of whether this opinion is correct, but rather how justifiable the opinion is.
The simple answer is "very". The thing that seems to baffle people about this is that they're conflating dominance with overall ability. The latter question is harder to determine, and essentially impossible when comparing athletes across sports. How could you possibly say Rousey is a better mixed martial artist than Valerie Adams (BI's #19) is a shot putter? But dominance has an objective basis to the calculation- results relative to the competition.
Who is more dominant than Rousey at what they do? LeBron is #2, and he's in a somewhat more difficult position in this matter since he plays a team sport. Truth be told, his presence on a team affects who wants to play there, which helped the positive results he brought to Miami and now Cleveland- that adds to his dominance of the NBA. He's overruled the team's coach, which might be shitty but can only be done by a dominant force. All the same, he may be the best player in the game, but he doesn't overshadow the game.
The two highest people besides Rousey in individual sports are Serena Williams and Usain Bolt. Bolt gets all his credit for blowing people away at the big events, but treats the smaller events as tune-ups he can lose; that doesn't make him less great, but if you're comparing him to someone who smashes everybody, his flat results don't quite match up. Serena is the one person who really has an argument here. Nobody goes a whole year undefeated in tennis, and Serena's death grip on the #1 spot- not to mention how long she's held it- is the stuff that will be legendary when we're all dead. If Rousey were beating her opponents in the third round- even in the third minute- Serena might well deserve to be ahead of her.
But that's not what Rousey is doing. She's beating the best people the UFC can throw at her so quickly people are throwing PPV money down hoping to see another full speed beatdown, which is not an expectation of any other fighter. Sara McMann and Alexis Davis weren't #7 ranked or something when they got their title shots, and Rousey ran them down. Zingano is the one person many observers (myself included) might give Rousey a physical struggle, and her determination to prove she could do that led to the worst loss of them all because Rousey's fight IQ is as high as her skill level. When Miesha Tate took her all the way to the third round, it was a clear strategy of not letting herself get beat rather than take any chances, and by the time the finishing armbar came it was clear Tate was flat out done with that fight.
She's also beaten the #1, 2, 3, 4, 6, and 9 fighters in her division, and it'll be #7 if she beats Bethe Correia. It's not entirely fair to say she's better because she has more wins against current top ten divisional fighters than anyone else, since every other division save strawweight and flyweight are much deeper, and fighters drop out of the top ten more frequently. But it does indicate the stranglehold she has on the division, since there aren't as many fighters pushing the current top ten out of their places.
This is where the difference between "best" and "most dominant" can be seen. Anyone who doesn't have Rousey in their top five P4P is an idiot, but #1? Set aside Jon Jones, since he took himself out of the picture, and there is zero reason to suggest she's surpassed Jose Aldo as a fighter. Demetrious Johnson is fantastic. Given the way Chris Weidman handled Anderson Silva in both of their fights- ie., not with a specific strategy like Chael Sonnen, but getting the better of him all over the place- there's an argument for him as well. But Aldo is less dominant because guys like Chad Mendes are much closer to his skill level than any woman is to Rousey's, Johnson has "normal" wins in a division not much stronger than Rousey's, and Weidman has no track record yet.
I suppose one interesting question is, will anyone overtake Rousey's apparent level of dominance before she becomes the clear #1 P4P? I'd bet on her hitting #1 first. But if Correia's game is as big as her mouth...