Freddy Kruger is the first character I think of when I think of the word horror. He's funny, scary and makes the film what it is. Robert Englund is Freddy and without him I don't think it would be the series that it is now.
Another person who springs to mind is Tony Todd. No matter what that man does, he's just so creepy! Loved Candyman and he was soooo kooky in the FD series.
Director wise, it has to be Wes Craven. Can't beat him. I've loved most of his films that I've seen and the Scream series is my favourite horror franchise.