Xaldin: definitely mainlining Strength/Agility, with Charisma as a side stat

Xigbar: hardcore Agility, some Int and Wis for his wibbly wobbly spacial warping nonsense

Rothe: Agility and Charisma, some Constitution every now and then

Flowey: UH.

Flowey is an evil GM

HE IS

I would think a lot of it is Int and Charisma

with Wis being his side stat

judging by the true pacifist route

Omega is just...insane

Chara: Agility, Strength, Charisma. would probably put some points into Int every once in a while.

scheming little shit that they are