I'm not convinced. Which doesn't mean I think he isn't the best, he might very well be, just that he hasn't shown it yet. Pep's clearly won a bucket of trophies, but lets look at the period before, under and after Pep at Barca (La Liga + CL results):
the 4 year period prior to Pep: 1st, 1st, 2nd, 3rd + 1 CL win
the 4 year period under Pep: 3rd, 1st, 1st, 1st, + 2 CL wins
the 4 year period after Pep: 1st, 2nd, 1st, 1st + 1 CL win
Better with Pep? yes, CL must rank higher than La Liga. Significantly? debatable. Did he fix a broken Barca at the time or was it more about replacing a fallen Ronaldinho with the best player in the world who just excelled at the Copa (2007) and the Olympics (2008) + Xavi-Iniesta combo? could be down to all kinds of things, the motions of other top clubs in Europe etc. Difficult to tell. Does this mean that Rijkaard and Enrique are close to as good as Pep? Apparently, they got nearly the same result with slightly worse squads, but probably not.
Comparing his time at Bayern (Before and under, since there's no after yet):
the 3 year period before Pep: 1st, 3rd, 2nd, + 1 CL win + 1 CL Runners up
the 3 year period with Pep: 1st, 1st, 1st + - (not even reaching the finals once)
Better with Pep? In the league - yes, Dortmund peaked (with Klopp emerging) during Heynckes period and reached the CL finals (that they lost to Bayern), they were the 2nd best team in Europe if you will and they won the league over Bayern before Pep joined. Enter Pep and Bayern didn't even reach the CL finals once and CL must rank higher than the league, however Dortmund declined, transfered big Lewa to Bayern etc so the only threat to the league subsided.
So he won buckets but not significantly more than his predecessors/successors. Which means it is impossible to deny that he is at least pretty good, but it is far from proof that he is the best imo. Mou, Klopp, Simeone etc did more with less at some point, so the only way for Pep to prove he is above them is to.. well do the same I think. Again, not saying this proves he isn't the best, it just doesn't prove he is.
Now he took City in the prem, yet again the team with the best and most expensive squad and bulked on another £150m net in players, (for reference that is bang on as much as we've spent on Sanchez, Özil, Mustafi, Xhaka and Cech). He -should- win the league comfortably one could say. Anything else is a disaster. And if he wins it narrowly I'd say it was the expected result with that squad and again it shows that he is at least pretty good but doesn't prove he is the best, to me at least.