Baked potatoes are the most nutritious as they retain the vitamins and are not cooked in fat. Microwaving has the same effect.

Steaming potatoes helps to retain the vitamins, but not to the same effect as baking, plus you have to remove the skins which is where many of the nutrients are.

By boiling potatoes you lose a lot of the vitamins and are just left with a pile of soggy carbs.

Roast potatoes keep their nutrients but are cooked in fat which makes them more unhealthy.

Frying chipped potatoes is very unhealthy, but oven chips are fairly nutritious as they still contain vitamin C, and are not cooked in fat.

Hope that helps!