26. Bad Wolf/The Parting Of The Ways 1.12-1.13
This regeneration story arch has some of its thunder stolen as a pivotal moment. It is technically the second Doctor and companion kiss if we count the Eighth Doctor and Grace, and the second time he chooses his companion's life over his own, the first being the Caves of Androzani. But it does however, lay to rest our questions surrounding Bad Wolf, the tying thread of the first series, and it creates Captain Jack's immortality, thereby laying the final piece of groundwork for Doctor Who's most successful spin-off Torchwood. It is also the first of several explicit occasions where the Doctor seems absolutely certain all things are hopeless, starting a trend in Modern Who where it is absolutely up to others to step in and save the day. But it would not have been a list maker if it were not for its closing seconds. As this Doctor prepares to hand the reigns over to a new man once more, he for the first time attempts to prepare someone, namely Rose, for what this process is, and what the change will mean.
25. The Ribos Operation 16.1-16.4
This serial is the beginning of the Doctor's involuntary quest to find all six segments of the Key to Time, a mini-plot arc which I recommend in its entirety especially if you find yourself fond of Miss Romanadvoratnelundar. Romana is entirely unique as a character as of this first meeting she is not yet a renegade, and is indeed perhaps a picture-perfect Time Lord student in some ways. Yet she is not much like the old politically-focused Time Lords we meet in Gallifrey-centered stories. From their very first conversation, the Doctor and Romana develop a style of bantering together which stays around throughout much of Romana's tenure on the TARDIS, including the habit where the Doctor merely repeats a wise decision Romana has made as though the idea were his own. This serial also introduces the concept of the Guardians, creatures even beyond the Time Lords who aid in protecting time itself. The White Guardian is in need of the Key to Time to stop time so it may mend itself, a curious process which although never explored again after this arc, likely paved the way for other memorable stories of a similar vein.