Second Doctor

Played by the late Patrick Troughton throughout, the Second Doctor holds a special place in my Whovian heart due to a number of firsts:

• First serial I watched: The Mind Robber (1992 repeat).
• First serial I rented: The Seeds of Death (back when video renting was a thing).
• First serial I purchased: The Tomb of the Cybermen (from Boots, VHS format).

Troughton arguably had the hardest role ever, in that his performance would decide whether the regeneration concept -- and the resulting change of lead actor -- would be a success. If he had failed to convince viewers that he was still the Doctor, albeit with a different (and in my opinion, greatly improved) persona, then the show may never have made it beyond Season Five.