I was stuck between seasons four and five, four had some of my favorites like Sacred and IMO the best finale. But season five has Reckoning and Lexmas among others so it got my vote.
The greatest science fiction series of all time is
Doctor Who! And I'll take you all on, one-by-one
or all in a bunch to back it up!"
--- Harlan Ellison, from his introduction
to the PINNACLE series of Doctor Who books