Only two really terrifying childhood memories:
The end of The Dead Planet, with Barbara screaming in terror at the sight of something evidently horrifying, but completely unidentifiable. [ When I got to see the Daleks in full in the next episode, it was such a relief that they were not whatever formless horrors had haunted my imagination for a week, that I loved them at once.]
The other is a man turning into a Varga plant in Mission to the Unknown. [Stupid as it sounds, I don't think I really noticed that none of the regulars was present, but their absence must have given the whole episode a less comfortable feel than usual. I remember it as very grim.]