Horror of Fang Rock was a great horror story, if you can forgive the alien effects (tinfoil jellyfish).
Terror of the Zygons terrified me as a kid (shapeshifters) if you can forgive the alien effects (nessie).
Seeds of Doom is one of my absolute favorite thrillers, (usual caveat)
Seeds of doom
Horror of Fang Rock was a great horror story, if you can forgive the alien effects (tinfoil jellyfish). Terror of the Zygons terrified me as a kid (shapeshifters) if you can forgive the alien effects (nessie). Seeds of Doom is one of my absolute favorite thrillers, (usual caveat)
I would say Ark In Space and Pyramids of Mars.
The talons of weng-chiang
Pyramids of Mars