Gaiamon

Digimon World 3
Gaiamon is assumed to be a Mega level Digimon, however it was never actually seen. It would have been created by the merging of Galacticmon with planet Earth, resulting in the extinction of the human race and the creation of an unimaginably powerful entity. Because this event never occurred in the game, nobody knows what this enigma of a Digimon looks like or the true extent of its power. It is also quite possible that he could have been the most powerful Digimon to have ever existed had he been created.