6. Evolution Of The Daleks
In this defining episode of Dalek mythos that is never explored again, Dalek Sec of the Cult of Skaro is fused with a human to make a Dalek-human hybrid in hopes of obtaining humanity's ambition and talent for war. Evolution of the Daleks is the second episode in a two-parter set in New York City in 1930, and the Daleks are using the nearly-completed Empire State Building as a collection tool for obtaining gamma radiation from a solar flare to power a machine to create more hybrids. The Doctor and Martha attempt to sabotage the machine before it's too late.
When He Should Have Regenerated: The top of the unfinished Empire State Building has been outfitted with three strips of Dalekanium (presumably the metal used to make the Daleks' iconic shells), and the Doctor needs to remove them before the gamma storm strikes the building with lightning. He only succeeds in getting one off before he climbs the mast and allows himself to be electrocuted so that his DNA melds with the Dalek DNA being used to create the hybrids. This later allows them to find a semblance of humanity, and ultimately save the day. However, the lightning strike is powerful enough to power the extremely advanced and utterly enormous machine and kill all of the Daleks' pig slaves, but somehow, the Doctor survives. Similar to the incident in World War Three, the electrocution is sustained over a relatively lengthy period of time, and courses through his entire body for the entire duration. The shock is much more powerful than in World War Three, so if he somehow should have survived that incident, this one should have been a sure thing in the death and regeneration books. However, the Doctor wakes up with a headache after a short period of unconsciousness.