Fact: There's no evidence the UFC has the best fighters in the the world.
People need to learn this: MMA has no competitive architecture. It doesn't have a world wide competitive system in which talent from around the world can compete with each other and escalate to an elite status. How do you become an olympian? You compete with the best athletes in your country, region and then the best athletes from other countries. You cannot be an olympic medalist sprinter if you have not proven yourself repeatedly to be amongst the fastest sprinters in the world.
How do you become a UFC fighter? You just need to be marketable. You don't need to be above any level. Anyone can fight in the UFC from a skill standpoint. From the bum with zero fights, to the experienced fighter. Now, that doesn't mean the UFC doesn't have great athletes. They do, but these athletes are judged based on performing in an extremely limited enviroment, against an extremely limited number of people, which the UFC selects primarily on marketability and nationality
How do you become a great UFC fighter? You beat other UFC fighters.
How do you become top ranked in MMA? You beat fighters from the UFC or Bellator.
Talent from anywhere else is pretty much invisible. It exists, but it's hard to find it without competitive architecture. Not to mention the UFC constantly ignores prospects and keeps mid tier, but marketable UFC fighters under contract. At the end of the day, the UFC is a business that sells entertainment. They don't need to have the best because their marketing makes sure everyone believes they are watching the best. And it works. They hyped Brock as the best HW on the planet after beating Frank Mir and people bought it, and so didn't the rankings.
Reality is, nobody really needs to fight in the UFC to prove anything from a competitive standpoint. The only thing you prove by being the best fighter in the UFC, is that you're the best fighter IN THE UFC. But of course, they will market you as the best in the world and rankings will do the same. However being the best in the UFC and actually proving you're the best MMA fighter in the world are two completely different things.
Without competitive architecture, you can be talented and fight other talented fighters outside the UFC and nobody will ever know or care. And people will most likely consider you unproven or a bum. Yet, what does UFC fighters prove to become UFC fighters? Nothing.