I say the best I've ever witnessed in the cage is a dude who did blow, took dick pills, drove over pregnant ladies, and never lost a single fight in the octagon. But he's an idiot who got caught multiple times roiding up so it's hard to call him the GOAT.
The true GOAT is a nice French Canadian dude who was smarter, stronger, and more technically skilled than everyone he faced and knocked off the number 1 contenders one by one while going undefeated for the last decade. He's only lost two fights, none since 2007, and he subsequently dominated the rematches against the two fighters that ever beat him.