Mainly because it scared the shit out of me when it was first broadcast and also the fact that I have been to the location of the cottage in the episode.
"Guardian of the abyss" would be a close second tho.
And "Children of the full moon" would be a close third!
Two Faces of Evil was a little too scary, whereas the first two I cited retained classic elements of country houses etc. Guardian reminded me of the 1968 film, The Devil Rides Out.
But I do tend to watch 'Visitor From The Grave' the most, I know it is rather predictable but I like the story and the ending is a surprise. 'Children Of The Full Moon' is another favourite. 'Witching Time' as well, mainly for the lovely Prunella Gee .
WITCHING TIME I adore also,followed by THE TWO FACES OF EVIL which scared me witless when I originally saw this when I was a child,not by the hitchiker or the possessed father and son but for some reason it was the hospital sister......