36. Dragonfire 24.12-24.14
I debated with this particular choice for some time before deciding that the departure of Mel and the arrival of companion-to-be Dorothy "Ace" McShane are notable for two reasons. First, Ace marks a rare occasion where the Doctor acquired a companion somewhere other than on his/her home planet. Her reason for ending up on Iceworld where she is found is most odd, as it is eventually discovered that she caused a time disturbance when her chemistry experiment blew up. From this early experience we discover Ace's intense and quirky personality, including her fondness for homemade explosive nitro-9. But I think what makes this serial truly notable is that in the end it is Mel who passes the torch to Ace, thereby directly picking out her replacement.
35. Earthshock 19.19-19.22
Notable mainly for the devastating companion's death that concludes the story, this story's main emotional conflict stems from the Doctor being so unwilling to give up his companions that he would rather start an argument than consider their requests to leave. While latching onto his companions is a trait present strongly in each of the Doctors up until this point, and even arguably afterwards, in no Doctor is it stronger than in this incarnation. It is also a chance to catch yet another version of the Cybermen which have changed significantly since their original introduction. Gone are the headlamps, replaced with a sturdier, stockier helmet and a reinforced chestplate. Gone is the exposed skin too, though these Cybermen, unlike varieties before and after, have visible mouths. Also, the course of the plot allows companions Nyssa and Adric to display their own talents as a slow shift towards a greater focus on companions themselves continues.