Not arguing with number of trophies etc., but a title of 'the greatest' takes into account other factors such as personality, character, what someone might have done for the game as a whole and not just themselves or thier club. So it will always be subjective and arguable.
Fergie has been manager of the richest club in England adn perhaps the world for his entire stint at Manchester United. He failed to win the league in his first 6 years i believe, but has had huge success after the premier league and all the SKY money started to be chanelled to the top clubs. Just coincidence?
Despite all the resources, the stability,the soft power, pretty much first choice of the top players only 2 Champions League titles in 25 years? Both extremely fortunate too. In cricketing terms he might be considered a flat-track bully.
There are many other great managers who didn't have the resources at Fergie's disposal and who left a mark on fans and supporters across club and even country divides: Bobby Robson, Jock Stein, Bill Shankly, Bob Paisley, Brian Clough are names that come to mind.
I'd go for Bill Shankly myself. He built the modern Liverpool from scratch. He didn't stick around for 25 years but he laid the foundations for the future success. He was also extremely charismatic. There are so many anecdotes about his interactions with fans, journalists etc. He was highly principled from all accounts and I don't think I've ever read a story where someone has had a bad word to say about him.