Who is the scariest Star Trek villian, or rather a better way to put it: who is the most emotionally disturbing? Who leaves you feeling uncomfortable after their villainous rant? Who makes you sit on the edge of your seat when they're about to do something bad? Who would you run for the hills away from if you lived in the Federation?
(The holodeck in here wasn't my idea, rather an amusing villians list. I'm sorry if I excluded your choice, I'm not very familiar with TAS, ENT, or some of the movies)