I'd be putting Hux in Slytherin and hating myself a little for it because Slytherin villains are such a cliché, but honestly the boy is the walking embodiment of all the things the decent Slytherins would rather everyone stop fixating on.
Ambitious, ruthless, clever but uses it for galaxy domination/genocide purposes, doesn't like to fight face to face but, according to Domhnall, would do more or less anything to win one if cornered, is possibly only going along with Snokes orders because he wants to be Emperor someday...
Plus you feel like in a full scale AU he'd be one of those pureblood kids whose whole family are Slyths going back generations.