TH: Yeah, he's the best. Sir Alex was a genius in his own right, but he coached in a much easier league with way less tactical variation. Sure, he had to contend with Arsene Wenger, Jose Mourinho and Carlo Ancelotti at various points, but the rest of the division was a bit meh.
Pep has put in the hard yards. It is worth pointing out, though, that Pep was handed pretty much unlimited resources. And yes, he worked wonders with them. But he did have every wish granted. No one else has had that luxury. If, say, Jurgen Klopp, had been handed that level of player from day one, this might be a different conversation.
RT: It's really tough to look past Sir Alex, but the fact that it is a discussion says everything you need to know. Pep is up there on the Mt. Rushmore, at the very least, which really is something considering how steady that Mt. Rushmore felt before he arrived. "Greatest" is a matter of perspective, and you could easily have a point of view that gives him that title. That is an achievement on its own.