There are some good criticisms in this thread, the doping and the 115 against City stuff and some other good points.
But one angle I've seen that is not valid is the whole "he never managed a small club and got them to overachieve" line. Of course he's not going to go to Luton Town or Cadiz after winning CLs. Literally no manager would do that unless they completely fall from grace and are forced to do that.
If a manager has to start at a smaller club and build up, that's a feather in their cap for sure (like Ferguson at Aberdeen or Xabi at Leverkusen this year) but if a manager gets a bigger club at the start like Pep and wins the CL, its absurd to think he has to go manage some "small club" to prove himself. It's like that silly "Messi never played at Stoke City on a rainy Tuesday night" line. There are valid critiques for why Pep is not the greatest manager ever, but "never managed a small club" is definitely not one of them.
True, the question has to be reduced to simply asking "has he ever overachieved with a team" and arguably he hasn't. He also never has failed at a job, but simply he does lack some success to consider him the GOAT for me. What did he do in his career?
Serial PL winner with the richest PL club - that's what you can and should expect
One time CL winner with the richest PL club - that's at best on par, if not slightly underwhelming
Serial BL winner with the richest BL club (which had won a treble before he arrived) - that's what you can and should expect
Never reaching a final with a treble winner but at least getting close to it - that's at best on par, if not slightly underwhelming
Serial La Liga winner in a duel against the biggest club in world football, with the core of the Spanish national team and the GOAT Messi - that's a very good result and probably it's fair to say he got them rocking much faster and better than expected
Two CL wins with Barca - very good result, no denying that.
So the only time he came close to truly overachieving was with Barca, but considering some things that happened (Övrebö, Doping allegiations, bribing La Liga refs) there are even asterisks to that.
In the end his most impressive success probably is that his teams never have a true off season, he is always delivering silverware. That's something no other manager can claim for himself.