I'll have to go with Invader Zim and possibly Gargoyles for this one. Invader Zim had creepy moments, like ripping out Keef's eyeballs and I still remember how creepy it was to watch Zim being turned into meat in one episode. Even though I watched this a few years ago when it was on Jetix on ABC Family, Gargoyles still was scary with its dark tone in the storyline and a villain like Xanatos. It was good and intense, which sometime lead to scary/creepy moments.