Most successful sure, but best manager? The ones who are seen as the best more or less also leave a legacy and a lasting influence on the game. Zidane won't do this as
@TheMagicFoolBus mentioned - he does not stand for a certain style, he just knows how to get the best out of the players available, but there is no vision or something like that. His team has no identity and no one will replicate their style because they just don't have one. And that is not a bad thing currently for them, it just means that Real will need a manager who can do the same on his own and who can not build on a certain legacy except the squad. But as we have often seen, managing a great team you inherited and keeping up their style and success is also difficult, so in the end this does not matter much, especially for Real who tend to have shorter cycles and often don't do long-term team building.