He's obviously a great manager but he inherited a pretty much the entire team Barca team that he managed. Then jumped ship after they peaked.
Took over a Bayern team that were already dominant domestically and looked set to dominate Europe with an extra push (he couldn't provide that).
Then moved to City who had a title winning team but still went all out preparing for his arrival including a complete restructure of their backroom staff. Then was then given an unlimited budget to buy his perfect team, then continued to receive unlimited funds to keep replenishing that team at will. Including buying players for top level transfer fees just to test them out to see if they worked.
Has he really ever experienced a challenge or had to overcome anything in his managerial career?
You can also make the argument that Alex was afraid of finding new challenges and stayed at Man Utd for his whole career where he had a complacent life with everything under control. During the period 2003-2006 Man Utd won nothing yet he still managed to keep his job. Or compare Alex to someone like Claudio Ranieri who had very little resoursces and won the Premier League (much bigger achievement than what Alex did in Scotland).
Claudio = least resources, won the least. But that Premier League title with Leicester is probably the biggest football achievement in the last 50 years, closely follow by Mourinho treble perhaps.
Alex = more resources than Claudio but not unlimited, still won a lot
Pep = Unlimited resources (but he didn’t spend that much more than other teams, at least officially). Right now slightly behind Alex, so Alex is still the best manager of all time. But in a few years Pep will surpass in terms of trophies.
There are arguments against and for all these 3, but none would say Claudio is the best amongst these three. Each person will have his or her own opinion. But in the end what will be remembered is the number of trophies won and the style in which they did it. And right now Pep is only slightly behind Alex, so I rate him slighly below. But that will change very soon (perhaps even by the end of this season if he can do a UCL and Premier League double), and I am pretty confident to say that.