It's common knowledge that wrestling is the best base to start with. But i wouldn't agree with the title of 5 champs being the wrestlers. It kinda implies that they're pure wrestlers and nothing more.
That is precisely why I qualified the post with Wrestling as a base and not the only fighting style. I think the fundamental advantage a high caliber wrestler has is the ability to instantaneously take the fight to the ground and change the course of the fight. This dynamic and explosive action throws opponents off while they attempt to adjust their game plan, whether that be a striker or BJJ guy.