In my opinion it's Louis Van Gaal. He had a system and he tried to implement a vision that for whatever reason it didn't work. But, for the brief time that it did, we totally dominated Spurs, City, Liverpool at Anfield and Chelsea at Stamford Bridge like I've never seen us do since. It didn't ultimately work because of poor recruitment, and because some of our players are too thick to learn and adapt, nor do they give a shit about double sessions or video instructions, but he was an amazing coach and I would loved to see him manage us earlier in his career.
Mourinho just bought better players and played pragmatic soccer without building anything long term. He left such a circus behind.
Moyes was just never the right fit for the club, he couldn't handle it.
I'd say LVG, OGS, JM, DM