Really depends on what it means for someone to be the greatest manager of all time. It's subjective.
I think in certain aspects, he is. He's probably the biggest tactical pioneer since Cruyff, and like 80% of elite managers are just copying him in one way or another. That matters a lot, whether you like it or not.
However, despite his genius, and the quality of players he's had at his disposal, there are asterisks next to the honors he won at both Barca and City...Negreira case + City's financial doping...it also really bothers me that he had no problems joining a sportswashing project created by such a state that Abu Dhabi is. Just totally wrong morally.
Not enough people care about this, or Newcastle, or PSG, or how one of Putin's oligarchs was allowed to buy Chelsea over 20 years ago.