The fastest method is the microwave in 30 second increments. My preferred method is to reheat them in a pan on the stove and add water, milk, or cream if needed for additional moisture. Another great option is to use an instant pot- place a rack on the bottom along with a cup of water, then set a pressure cooker safe bowl of potatoes on the rack and pressure cook for 5 minutes. The steam keeps everything fluffy, and you can walk away from it.