Best is always subjective since there are just too many metrics of comparison, unless its something like Ten Haag vs SAF/ Ancelot. There are in fact people believing Tuchel is better than Ancelotti. To Man United fans, even if there Pep or Ancelotti win 10 trebles in a row, the best manager would still be Alex Ferguson because he did it at Aberdeen. But if one day either Pep or Ancelotti join Man United and win just a single PL or CL title after 20 years + for them, then they might move ahead of SAF.
This is not a dig at you guys. Its perfectly normal. Just like Brazilian claiming Pele is the best ever over Messi. Ask Man City fans who the best manager of all time is and they would all say its Pep.
Personally, as a neutral fan, I would say Pep is the best ever or at least the best of this century. To me trophies only matter to an extent but I have a broader view - the influence on the sport is more important. The way teams player football nowadays or in the next 10 years is influenced by Pep. Yes, there will be asterisks around Pep’s achievements (referee bribery, 115 charges…) but even with half the trophies Pep is still the best ever due to his influence. To talk about his influence you also have to consider his students. Half of the most promising coaches are Pep’s students: Alonso, Xavi, Arteta… SAF’s students include Roy Keane, Gary Neville, Rooney,… Ancelotti doesnt even have any student or anyone influenced by him. In a hypothetical world without Ferguson or Ancelotti, football would still be the same. If its not Man United to dominate the Premier League during the 90s and 2000s then it would just be another team, probably Arsenal. Similarly with Real Madrid and AC Milan. You cant say football would be the same without Pep.
Below Pep are SAF and Ancelotti (counting only managers of the 21st century). Mourinho is slightly behind.