Probably a speed-lowering one. If Shell Side Arm ever decides that physical would be stronger (like against a Chansey) it will deal more damage than if you were Modest or something.What nature would be the best for a galarian slowbro??
No, it does not pick from your highest attacking stat. Nor does it pick from your opponent's defending stats. It compares Att->Def and SpA->SpD and picks the stronger one, though it doesn't take into account some effects like screens.