26. The Invasion (Second Doctor)
Just as the first Doctor had to watch his worst enemy - the Daleks - invade the Earth for the first time, so too the second Doctor had to watch his worst enemies - the Cybermen - invade Earth. This epic 8-episode story also introduces UNIT, who would go on to feature heavily in the series for many more years to come. Episodes 1 and 4 are both missing, but the audio remains, and both episodes have even been animated along with the existing audio track, so it still makes for a unique and pleasurable viewing.
25. Father's Day (Ninth Doctor)
One of the earliest episodes of the revived series of Doctor Who, Father's Day tells the story of what happens when the Doctor takes his companion, Rose Tyler, to 1987 so she can meet her father before his death. Problem is, Rose manages to prevent her father's death, which in turn unleashes Reapers on the Earth. While some criticized the Reapers, the heart of the story received unanimous praise, and further praise for the acting of its three leads.