The Second Doctor Who
The Second Doctor was played by Patrick Troughton and first came to life in fourth series episode, "The Tenth Planet", as the first Doctor grew progressively weaker battling the Cybermen.
Patrick Troughton brought a different style to the series and was nicknamed "The Cosmic Hobo", as he appeared far more scruffy than his predecessor. He also appeared to be quite bumbling and panicky in the face of danger but always acted heroically and morally correct.
More than any other perhaps, this incarnation of the Time Lord was a wolf in sheep's clothing.
This regeneration marked a turning point in the series and marked its longevity. With the arrival of a younger Doctor, the style of the show changed to a faster pace and while there were still the regular enemies like "The Cyberman" and "The Daleks" as well as new enemies like "The Great Intelligence" and "The Ice Warriors" there was a preference towards a different "Monster of the Week".
The Second Doctors time came to an end when he was put on trial by the Time Lords for breaking rules of non-interference. Despite the Doctor's argument that the Time Lords should use their great powers to help others, he was sentenced to exile on 20th century Earth, the Time Lords forcing his regeneration into the Third Doctor in the process.