Very talented footballer. But very flawed person so didn't achieve as much as he could / should have. As well as the injury issue of course.
I wouldn't put a label on him, or any other player, as 'the best...' But he was up there for consideration talent / potential wise. However he was weak when it came to the mental / professional side of the game and that's also a key part to consider in terms of the all round ability of a player.
As regards could Fergie have got him to fully settle down and focus on his game? No, I really don't think so. Not permanently. No one has - not himself, friends, family, managers, teammates, medical professionals, etc. He seems to be too troubled for anyone to get him straightened out permanently.
Fergie was very good at it, but he wasn't a miracle worker. He may have managed it for longer than most, and we'd have benefitted from his quality performances for a time. But I fully expect it would have ended at some point with Fergie deciding Gazza had become more trouble than he was worth in terms of regular off the pitch issues / poor influence on youngsters, etc.