Originally Posted by
Heteronym
H.P. Lovecraft gets my vote, although I also have an affection for Arthur Machen.
Lovecraft took horror away from the 19th century tropes and opened it up to cosmic, almost infinite possibilities. His imagination operated on a scale that Poe and Maupassant, with their little ghosts and vampires, couldn't even comprehend. He created mythologies and a parallel, secret history of Mankind and the world. That's a remarkable feat of the imagination.
Poe followed trends from Europe, mainly Germany and the English Gothic. Lovecraft created his own tradition.