The water heater. By a wide margin the most expensive. This is the one surface in your property whose entire job is transferring heat into water — which makes it the primary site of scale deposition. On a gas tank, scale settles on the bottom as sediment, sitting directly between the burner and the water. The burner runs longer for the same result, the tank bottom runs hotter than designed, and the popping sound is water flashing to steam beneath the crust. On an electric tank, it buries the lower element, which then burns out early. On a tankless unit, it coats the heat exchanger, restricting flow and triggering error codes — and descaling is a warranty condition on many models.
In all three, you're paying more energy for the same hot water, every single day, and shortening the equipment's life while you do it.