One of the great things that happens with a long-running series, be it a comedy or science fiction or a drama, is that recurring characters can be created and evolve over time, earning larger, longer stories than they might otherwise. We don't even need to see them terribly often to know who they are, what they are, and what they bring to the table. Doctor Who has created a sizable number of recurring villains through the years—the Cybermen, the Daleks, the Silurians, and the Master to name a few—but perhaps my favorite recurring villain is Davros.
Originally portrayed by Michael Wisher in "Genesis of the Daleks," (a 1975 story with Tom Baker playing the Doctor), Davros is the mad scientist who transforms the Kaled into the Daleks. It is Davros' endgame (mostly) to use the Daleks to subjugate the rest of the universe so that Davros can control everyone and everything. Typical bad guy stuff, if you will.
Clearly it isn't this typical evil ambition which separates Davros from your average recurring baddie, it's his place as the creator of the Dalek race. Here are the Daleks, the ultimate evil force in the universe, the baddest baddie that there is, and not only did Davros create them, but he's happy about it. When the Daleks fail to win, it is a blow which Davros takes personally. It is one thing to invent something horrible and be remorseful about it, it's quite another to applaud your creation, and that's who Davros is – unrelentingly evil and horrifically ugly to boot.
Davros' appearance in the Fifth Doctor (Peter Davison) story "Resurrection of the Daleks" is, I believe, his third appearance in the classic series, and it is one which clearly builds off the first two. One need not have seen "Genesis of the Daleks" or "Destiny of the Daleks" to watch "Resurrection," but it can't hurt, especially as it is the war which takes place in the second appearance, "Destiny," which leads to the crisis faced by Davros in the third (although the specifics of said crisis have their beginnings off camera between "Destiny" and "Resurrection").
Davison may not have been the Doctor for very long (a mere three seasons plus the odd special… and "Time Crash" is an odd special), but his appearance here with Davros is exceptionally notable. It isn't just that the end of the episode features (this would kind of be a spoiler, but we are talking about an episode of a television series from 1984) the departure of a companion. No, "Resurrection of the Daleks" shows us the start of a major fight between groups of Daleks. Plus, if I can say this without sounding too much like an Anglophile, it's a cracking good storyline.