He's never stayed long enough at a club to rebuild a team and stay successful like SAF did. Infact there's quite a lot of things Fergie has done that Mourinho hasn't but I don't think anyone doubts that Mourinho is one of the top managers in the Premier League era.
To be fair I don't think that necessarily needs to be a hallmark of being one of the best. On the other hand, switching from club to club and gaining instant success is also a feat to behold. Ferguson made United the brand/club they are today - one of, if not the, biggest club in the world. Which is why I rate him as the best.
SAF was the best manager the PL has ever seen. Just look at what he won with that shower of ****e United team that Moyes has, how they won the league last year is beyond me.
You may have just sealed that argument!! If you win the league with Ashley Young and Cleverley in your side, you're doing something pretty remarkable!