8. Tomb Of The Cybermen
This may not be the story that introduces the Cybermen, but it uses them more effectively than almost any other story. It's a classic base under siege story, that has the Doctor and his crew exploring an abandoned tomb with a bunch of nefarious types. The relationship between the Doctor and Jamie is pitch-perfect, and Victoria makes a delightful addition to the group (such a shame that this is her only intact serial). Patrick Troughton is firing on all cylinders here, from his banter with Klieg to the subtly heartfelt conversation he has with Victoria about remembering his family. And the reveal with the Cybermen climbing out of their tombs is so eerie, so clinical, that it's probably one of the most chilling moments in all of Classic Who. Fans can argue that this isn't the strongest story, but I think that it serves as a definitive serial of what Doctor Who is.