Scariest Who? That's a tough one... like Nos42, I was also a little freaked out by the Autons with Spearhead from Space and Terror of the Autons being one of my first Who televised stories that I watched when I was about 6 or 7; I think I had trouble sleeping for a few nights after those!
My brother (who was 3 years younger than me) also got freaked out by the Daleks and presumably Davros in Genesis of the Daleks when he was roughly the same age and as a consequence wouldn't watch any Who involving Daleks for years afterwards.
Nu-Who is also extremely terrifying; but for completely different reasons...
