16. The Caves Of Androzani 21.17-21.20
This serial is a truly epic regeneration tale which sees the Doctor poisoned to the point of neural shut down, but holding off regenerating long enough to not only find and rescue companion Peri Brown, but also trek through the an oxygen-deprived cavern to get an antidote for this poison to hopefully save him and his newest friend, only to discover there is enough for just one of them. Then, the Doctor is forced to carry the half-conscious and still recovering Peri across a desert to the TARDIS. There, when we think his worries might finally be over, he has a sort of hallucinogenic personal retrospective on the journeys and acquaintances he has had in this particular body. He even expresses a worry that he may have waited too long and may no longer be able to regenerate. It is not a much better trip for relatively new companion Peri, who in addition to also being poisoned and jailed is subjected to unwanted amorous advances from a lonely damaged mutant, Jek. Peri is well aware of the immense sacrifice the Doctor makes to save her life, making this regeneration all the more heart-wrenching. But this awareness also helps explain why Peri is willing to put up so much from the less-than-kind version of the Doctor which follows. This comparatively brutal and tragic transition from one Doctor to the next has no equal in any of the regenerations we have seen in combining this level of both physical and emotional suffering.
15. The Deadly Assassin 14.9-14.12
A rare opportunity where the Doctor is on his own and without a companion proper either currently on board or about to join up. Indeed, this is probably the only story of its kind in all of Classic Who. The story, which is largely a grand battle of wits between a dying version of the Master and the Doctor, sets up the groundwork for the Master's own personal battle to find a way to keep living on. The Doctor's actions here further entrenches the spirit of animosity between them. It is also notable for cementing the idea that Time Lords get 13 lives, or 12 regenerations each, but given this plot by the Master, also arguably leaves open the door to find loopholes. It is a chance to see some more of the people of Gallifrey who generally paint themselves in a very poor light they are impulsive, vengeful, paranoid, and power-mad for the most part. They quickly resort to violence, even torturing the Doctor, all out of fear that someone may invade if they do not maintain a façade of order. Worse yet, they are almost always defeatist. Taking all this into account, it's no small wonder the Doctor and Master never fit in there.