You can't go wrong with the Hinchcliffe era, ie Tom Baker's first three seasons. That's essentially the golden period of the series. Not only recommended; more like required viewing.
At the age of five, Skagra decided emphatically that God did not exist. This revelation tends to make most people in the universe who have it react in one of two ways - with relief or with despair. Only Skagra responded to it by thinking, 'Wait a second. That means there's a situation vacant.'