Most frightening character in Naruto or Naruto Shippuden?

Anko.

The way Yamato will look at you

Hidan. It was a terrible experience watching a joinin like Asuma's death. Hidan was immortal and also that sacrificing curse technique was invincible. It was really frightening to see his abilities. I was also worried about Shikamaru. But..later Shikamaru proves that having a sharp brain is better than being immortal.