The Invasion (Second Doctor).
An excellent addition to the Troughton's line-up and probably my favorite Second Doctor adventure now that I've seen all his existing serials. All of the characters had something to do and none of them felt superfluous. Removing both Jamie and Zoe for an episode apiece probably helped as it allowed other characters to be fleshed out. The plot was also strong enough that it did not feel stretched out over it's eight episode run.
UNIT's introduction was excellent and I can see why the producers decided to make more extensive use of the organization during Pertwee's run. Of course major props must go to Kevin Stoney for his portrayal of Tobias Vaughn. He's perfectly calm one moment and bordering on insanity then next whenever Packer screws up. An excellent villain for the Doctor.