I used to be of the mind that any manager guarantees success with certain football clubs, but my opinion has changed over the years when you look at what managers like Zidane achieved at Madrid, only for it not to go the same way when he left, same with Barca once Pep left and so on and so forth. Managers are important, however there are multiple reasons Pep will never be the best manager ever.
Firstly I would argue that he's probably the best tactician that's for sure, and the unorthodox ways he makes use of his players and they don't look out of place is quite a talent to have and you rarely see it if ever... however
Barcelona - Had arguably the worlds best club side of players at the time, probably the best player to ever play the game, arguably the best midfield trio to ever play the game and so on and so forth
Bayern Munich - Have just won the league for the 10th season in a row, cherry picks all the best German talent so their rivals can barely make a competition out of it, including players like Lewandowski in his prime. There was always a feeling he underachieved here despite everything though.
Man City - Financial doping, any amount of money spent on any player he ever wanted, completely unlimited funds and the ability to replace bad signings with a brand new shiny one the next season which means his squad depth is unreal and you virtually have world class players in all positions and even on the bench.
Where's the adversity? Again he has to make something of them of course but he's had virtually everything handed to him on a silver platter at every club he's been at.
As fantastic as a manager he is he will never beat the likes of Sir Alex, Busby, Mourinho, Clough, Shankly etc
These managers had different types of challenges throughout their careers, they didn't always have the best squads, the most money and yet they were able to reach incredible heights
Sir Alex with Aberdeen and constant success with United, first team to do the treble and with loads of academy players, winning league titles with some abysmal squads
Busby - Winning and playing attractive football with young academy players, winning the European cup after the Munich tragedy
Mourinho's work with Porto, Inter Milan, to an extent Chelsea and Real Madrid, perhaps the only manager to take it to Pep for a while
Clough's incredible work with Nottingham Forest
In an ideal world in order for Pep to be considered amongst the top echelon of managers he should have to step outside his comfort zone, manage a team that isn't the best in the world with unlimited resources everytime and make something out of that, but he's never going to do it and instead he'll pile up accolades with his unlimited resources and be considered one of the top 2 managers of all time, it's stupid