Ununoctium is the heaviest element that is man made. Uranium is the heaviest naturally occuring element though. There are various factors that might determine heavy, however. When it come to having the highest density, the elements osmium and iridium take the cake. The element that has the most atomic weight is oganesson.
Like many things in science, the exact answer depends on the exact question. Defining heaviest can be tricky. The heaviest in atomic weight is one answer and the heaviest in density is yet another. That is why science is known to be specific and factually based because one has to specify everything in order to come up with the exact right answer.