Since I’ve yet to go back and watch all of the pre-9th Doctor serials, I’m only counting the 9th Doctor incarnation onward. One from each of the six seasons/series, and multi-part stories will count as one.
Season 1: Father’s Day
Season 2: The Impossible Planet/ Satan’s Pit
Season 3: Blink
Season 4: Silence in the Library/ Forest of the Dead
Season 5: Vincent and the Doctor
Season 6: The Impossible Astronaut/ Day of the Moon
What are your favorites?