Which nature are you going for? In order of coolest, i think it's Timid or Modest (Scarftoad KILLS), Bold (Defensive is great, too, Calm (Specially Defensive), Relaxed (ALWAYS makes sure it gets weather going)!
Hidden Power Grass could also be good, but in all honesty, Politoed can run Hydro Pump/Focus Blast/Ice Beam and Perish Song or Hypnosis--even on an offensive set!