46. Mawdryn Undead 20.9-20.12
Most important for being the first appearance of long-time companion Vislor Turlough, who is given the unsavoury task of killing of the Doctor by the Black Guardian, the pure evil energy last seen in the previous Doctor's tenure. Separated from the Doctor are follow companions Nyssa and Tegan who have problems of their own dealing with a truly grotesque alien Mawdryn who claims to be an injured Doctor. This serial also sees another return of the Brigadier, who is able to be an integral part of saving the day one more time. It could be very well considered the Brigadier's truest time as a companion proper.
45. Castrovalva 19.1-19.4
This first serial of the Fifth Doctor is notable even before because of events immediately proceeding the regeneration, where the Doctor has already been aiding his former self and his companions as a mysterious figure known as the Watcher. This establishes that the Doctor can exist corporeally between regenerations. It is also interesting for this Doctor's particular brand of regeneration sickness, which allowed for some really great comedic moments, even involving the Doctor parading about in a wheelchair. It allowed for some great bonding among companions Nyssa of Traken, Tegan Jovenka, and Adric. Finally it expanded Time Lord powers and sciences to new degrees, as the Master is able to manipulate the existence of an entire planet, as well as remotely affect the Doctor's TARDIS and her information banks.