Actually, Mousse is consistently the biggest asshole of the three. Kuno is second. Tarou takes the cake, despite being a two-volume wonder. Ranma's assholishness is minimal when stacked against his castmates.
Likewise, Kodachi's assholishness is second to none, and Shampoo runs after her. Akane's not last, but she's not very high on the list either.
The trick is, they're ALL major assholes, so a comparison grading scale's too difficult to make. Kind of the point for the pioneers of tsun, yan, and dere.