Re: Best Season-long Story arc
The "Entopy" arc is really called The Master trilogy, since it contains the three stories that saw the return of the Master. The Keeper Of Traken, Logopolis and Castrovalva. The 18th season also had the E-space trilogy consisting of Full Circle, The State Of Decay and Warrior's Gate.
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