For creepy and scary, not enough credit goes to Sephiroth, in my opinion. Go listen to his theme, Those Chosen By The Planet (would link but I don't have enough posts right now) or Anxious Heart (what I imagine his effect on Nibelheim was). Also, his near Godlike powers (as seen from the Player/Cloud's perspective) and control over your own actions versus your inability to control yourself (at the lost city, disc 1, or the temple of the ancients, possibly also disc 1).
As hokey as walking into the flmaes may be, to a young person being introduced to a genre, it was a powerful image.