At City, Guardiola built one great side. At Liverpool, Klopp built one great side. At Arsenal, Wenger built one great side. At Chelsea, Mourinho built one great side.
Guardiola got burnt out at his only truly great side and had to leave, and may do so again. Klopp ran out of steam and had a relatively poor final season at Dortmund and hasn’t won the league yet. Wenger stuck around as his side got increasingly worse quality wise and never won another title. José left, returned and couldn’t sustain even close to the same dominance before going into a tailspin.
Alex Ferguson built multiple great sides and won numerous titles in the process. No burnout, no spirals, no fading away and no sad returns.
He lost battles across the years, but he never lost wars, and he left a champion, after a serious lack of investment for his final 6 years. He still managed to win his final war against his final mega rich adversary.
He was a phenomenon, and while the other coaches in that list can be proud of the teams they have built, none of them have even come close to achieving what Ferguson did.
It’s like trying to argue that Neymar is better than Pele. It only makes you look stupid.