Right now I would say Michael is the current most popular horror icon, at least out of the more modern ones (not even going to attempt to include the likes of Dracula, Wolfman, etc.). Freddy and Jason were both probably more popular than Michael in the '80s and early '90s when they were at their peak popularity but given the Halloween franchise has endured for longer and has quite a few more recent movies than either the Nightmare on Elm Street or Friday the 13th series, I'd say he doesn't have much competition in that department.
Admittedly there's more potential with characters like Freddy and Jason because they lean way more heavily on the supernatural than Michael. You can be a hell of a lot more inventive with kills with a character like Freddy who has an entire dream world at his disposal. That said, I think what makes Michael Myers so enduring with audiences is that, especially in that original movie, he's someone who could believably exist in real life. When I first watched A Nightmare on Elm Street as a kid, I was scared but in a fun way. When I first watched Halloween I didn't sleep at all that night just because of how realistic it was.