I reckon it boils down to what is more impressive, Canelo becoming the undisputed champion at super middleweight or Usyk successfully becoming unified champion at heavyweight.
I think it is more of a shock that Usyk defeated Joshua than it is Canelo becoming undisputed so I would say the greater accomplishment has to be the Ukrainian.
So in my mind Usyk is number one and Canelo is number two, and who number three is is where the real debate and argument comes into play.