The heaviest naturally occurring element with an atomic number less than that of other elements is uranium, with an atomic number of 92. Uranium is a naturally radioactive element and has the highest atomic number of all naturally occurring elements. It is commonly used as a fuel in nuclear reactors.
However, if you are referring to the heaviest element ever synthesized in a laboratory, the answer would be different. The heaviest element created thus far is Oganesson, with an atomic number of 118. Oganesson is a synthetic element that was first synthesized in 2002 and is highly unstable, with a very short half-life.
It's worth noting that scientists continue to explore and create new elements through nuclear reactions, so the information about the heaviest elements is subject to change as new discoveries are made.