Depends on what worries you. Stainless steel is the least reactive, but higher maintenance. Cast iron, carbon steel and aluminum are all reactive with acidic fo

Depends on what worries you. Stainless steel is the least reactive, but higher maintenance. Cast iron, carbon steel and aluminum are all reactive with acidic foods and will leech metals -- plus the acidic food will strip all the seasoning off. I cook with lots of tomatoes, so that rules out all of these for me. Cast iron and carbon steel (to a lesser extent) need to be seasoned to not stick like crazy and to not rust, which involves polymerizing cooking oil onto the surface. The health effects of this polymerized oil is unknown and the oil smoke is not great for indoor air quality. Digesting regular amounts of rust is also another unknown health effect, though iron can accumulate in people with certain genetic disorders. I think the safest option is stainless steel, and funny enough, Teflon. They are both inert and Teflon is only toxic if you cook on high heat. I keep a Teflon around solely for scrambled eggs and fish on low heat, but they are very doable in a well oiled stainless fry pan as well.