I don't have very strong opinions on who the greatest manager of all time is, like with players, I just know who I don't think should be included.
Do you know what's funny with managers? People say that to be a great manager, you need to have a "distinctive style" but then they also say "you need to have a Plan B."
Carlo structured his tactics and style to the players that were in front of him, he didn't demand that teams become restructured to please him--and you can say, "oh that's why owners like him"--but it's also why players loved him: he was loyal to them, he knew how to get the best out of them.
You could say Carlo should have won more, at Milan in particular, because that team from 2002-2007 was the best team in the world, and no one was close, it might be one of the best teams ever. But before Carlo came, that team was largely there, and was mediocre in comparison to what it became, so maybe his skill was in how he was able to extract the most out of players and that is his special skill rather than some tactical choice?