The easiest answer would be water/anything. You have Water/Ground, Water/Grass, and Water/Steel to wall the ever living shit out of unsuspecting trainers, and then stuff like Water/Ghost, Water/Flying, and Water/Electric for the offense when the trainers team is worn out.
My theme badge would be a tidal wave, to represent the power of the sea.
If we're talking about just one Dual type, I have to go with Water/Ground then. Quagsmire, Swampert, Wiscash, Gastrodon, and Seismatoad, almost a full team of rape.
