You can only compete against those that are put in front of you, therefore Dorian was the most dominant Mr Olympia.
He competed in the toughest era - Levrone, Shawn Ray, Wheeler, Nasser, Dillett, etc, were all at the peak of their careers, and he failed to lose to any of them, with a torn bicep to boot for most of the time. That's dominance.
Haney only had to compete against midgets like Gaspari and Labrada. The 80's was a poor era for bodybuilding.
Ronnie was beating older versions of the guys that Dorian beat in the twilight of their careers. And there were several Olympias he shouldn't have won - Martinez should have won one year and Levrone had him beat in another. It could also be argued that Flex should have won in 98. Plus his legacy is tarnished when he should have retired in 2005 after the 8th win. Instead his physique went to ratshit.
Arnold had to beat Oliva which was difficult but other than him there was little other competition.