I wouldnt say he is the best manager in the world. I would say he was in the mid - late nineties.
there were two things that turned united into a great team.
The Youth Academy which bred, scholes, beckhams, nevilles. etc. This has not been repeated and probs never will.
And Fergies management of the team during these times...
Anything in the early noughties can surely not be classed as greatness as neither can present day as he had a vasts amounts of money to buy world class players which in fairness takes having to be a better manager down a notch or two. Games can be won alot more easilty when you have 6 - 7 £20 + Million pound players in your squad.
But he was a great manager.
Now hes just a manager with great players..