If the movie was accurate to the book, then I would say Pennywise is the GOAT slasher villain.
...because the movie would have been just as much a slasher, as nearly all popular slashers are considered so today.
But here is why I still stick with Pennywise. Even though the made for TV mini-series had to be toned down significantly, the story and characters still made an incredible statement to how powerful the story was. Just imagine if "IT" were made into a real movie. I love Freddy(the original Robert Englund version, not the remake Jackie Haley, who is only so-so), and F13 is always going to be awesome, Pennywise would trump them all, given a fair chance. I *almost* think it's impossible to give IT a fair chance, because the film would traumatize kids pretending to be tough, to an entirely different level.