4. The Tenth Planet 4.5-4.8
It's hard to tell what is more historic about this serial, the first appearance of the Cybermen and their planet Mondas, or the very first regeneration of the Doctor. These first Cybermen still have exposed human flesh, their hands are not yet part of the conversion. They also sport a more flesh-like outer shell, have head lamps, and walk with a more natural, human gait than their more modern cousins. There is a real sense of tragedy in this one. You grow to feel bad for the Cybermen, who want so badly just to survive and save their planet, even though no one wants them to kill humanity to do it. You also feel for the humans caught in the immediate crossfire, and cheer companions Ben Jackson and Polly Wright's efforts to help, which makes you forget why the Doctor's not doing more. And then it hits you again, he is dying. The regeneration itself is rushed and only occurs at the closing moments of the serial, giving you a hint of that same confused feeling you would have had watching it for the first time. A truly brilliant end and beginning.
3. An Unearthly Child 1.1-1.4
If it's not clear why I am recommending this one, you are certainly reading the wrong list. It makes the list for being the first serial and is on the list, primarily for that basis. It introduces us to the first companions, the First Doctor, first trip in the TARDIS, the first whoosh of the materialising sound...you get the point. Were it not for its status as the inaugural serial it is not worth much of a mention. The Doctor's granddaughter, Susan, and companion Barbara Wright spend most of the serial displaying their signature shrieking, the man's man attitude of companion Ian Chesterton is appalling, the wildlife noise of prehistoric earth sounds like a puppy whining, and the 'cavemen' who are the focus of most of the plot are absurd. I do recommend sticking it out to see the wrestling match toward the very end of the serial. Good comedy, there. Still despite its faults there is something to be said for knowing where it all began and getting a true sense of just how far the institution has come in fifty years of space-time travel.