What's the creepiest manga you've ever read, and why? For me, it's gotta be this manga I just finished called Uzumaki: Spiral into Horror. It actually had me shook at some moments, especially in the early chapters.
Another one that turned out rather creepy to me was Narutaru, AKA: Shadow Star. Even though that turned out to be more... uh, highly disturbing/confusing than anything else.