There is no clear and simple answer to this question.
However, many people will claim that it is the Maliwan Hellfire. The Hellfire is an SMG (Sub Machinegun) which shoots x4 Fire Elemental bullets guaranteed and has a chance to create splash fire damage. In my opinion the weapon is completely overused and overpowered. There are many other weapons which are powerful as well, but I would say that is the most well known next to the Volcano.
Fire seems to be the most popular element due to the fact that it is very effective versus flesh, and there are many flesh enemies in the game. Next to fire corrosive and explosive seem to be about tied.