Who the best is, is utterly subjective, I hate people trying to name the best players, because its so completely pointless, if I tried I can't quantify who I think the best is, what film I like best, what food I like best, etc, etc.
What I'd say about Fergie is, he's been a great manager, I don't think he's currently anywhere near as good as he was 10 years ago, I think the team he's built now isn't even a patch on previous teams he's built. I think he's got incredibly motivational skills and can get more out of a particular group of players than most other managers could. I'm not convinced he's the best buyer of talent in the world based on the number of high cost flops and how many people were after certain other players like Ronaldo... its not like Fergie saw something there that no one else did.
He's one of the most successful managers around, but has had vast resources during his entire reign compared to the competition around them, it doesn't negate what he's done, but mistakes don't cost him as much as they'd cost other managers, and few teams can compete on size of squad or ability to attract top players, which are both huge advantages for any manager.