I voted The Empty Child / The Doctor Dances.
The first time I heard "are you my mummy" was proper scary - like the show was for me when I was 8 years old.
The ones that really spooked me as a kid back in the day were The Green Death and Terror of the Autons. Nightmares.
For me, Blink. The Empty Child close behind along with Silence In The Library (disappointed that one didn't make the shortlist).
I may have to watch them all again to refresh my memory.
I didn't watch very many of the original series.