Aluminum.
Finally, you might consider an aluminum boiling kettle. Aluminum is rarely considered as a brewery material, which is a shame. Aluminum is cheaper than stainless steel and also conducts heat better. Commercial breweries do not use aluminum because of its incompatibility with "clean-in-place" (CIP) chemicals such as sodium hydroxide (caustic soda). Aluminum is an option for home brewers, however, because they can scrub rather than resort to reactive chemicals. With proper care, aluminum pots should last quite a long time. They can be found in many sizes at restaurant supply stores.