It is an inconvenient truth that Bill Shankly did the turnaround at Anfield (appointed 1959, on the 2nd division, won the first league title by 1964) and then the transition to his second great team.
Bob Paisley built on this foundation and sustained success for 9 years, he was a good manager, it could have fallen apart very quickly after Shanks retired if his successor had not been capable.
Sir Alex took 6 years to do the turnaround - I'd argue that it was a harder job, as United were a mid table to top 5 team and the burning platform for change was less compelling than for a big club in the second division (no disrespect to Shanks, who did a great job and built a proper coaching team around him.)
Sir Alex's track record speaks for itself and is difficult to compare directly to Shanks or Bob Paisley, as they all operated in very different times . All stand head and shoulders above their rivals, with the possible exception of Brian Clough, who achieve amazing results on a shoestring, although over a shorter period of time at each club.
I'll leave the last word to Bill Shankly, who said publicly that Sir Matt Busby was the greatest football manager, a statement that says as much about Shankly's generosity of spirit.