According to the search results, the most radioactive element is a matter of debate. However, two elements are often cited as the most radioactive: polonium and radium. Polonium is a naturally occurring element that releases a huge amount of energy, and its most stable isotope, polonium-210, has a half-life of only 299 nanoseconds. On the other hand, radium is the most radioactive natural substance ever discovered, and its most stable isotope, radium-226, has a half-life of over a millennium. Therefore, it can be concluded that radium is the most radioactive element with the longest half-life.