The Green Death
For me, The Green Death is not only one of the best UNIT stories, but one of the best Doctor Who stories ever. It has so many elements in it that contribute to its richly-deserved status as a classic.
Having said that, I'll start by talking you through the story's detractions. The camera-work during the "boating through the maggots" sequence is a little shoddy, with the principal characters and foreground so obviously cut into other shots that the sequence becomes completely unbelievable. Similar problems are encountered when the Doctor and Sgt. Benson attack the maggots. However, Doctor Who was never about superb effects, but about the story.
And so that's where I shall move on to. It is, unfortunately, one of those Doctor Who stories, like The Tomb of the Cybermen and The Caves of Androzani, that has a blatantly obvious moral. Now, don't get me wrong. One of the best things about Doctor Who is its moral undertones. It tries, and often succeeds, to teach us important lessons and principles. But it really grates with me when the moral undertones become,... well, moral overtones, so obvious are they. There are also some important story issues. For example, when the maggot in the Nuthutch is found dead- next to Professor Jones' special fungus, which it has been eating- the Doctor fails to make the connection between the fungus being fatal to the maggots, and the fungus being the basis of a vaccine against the maggots. It's an almost unforgivable scripting error in the sometimes almost omniscient Doctor.
But, I must now go on to my reasons for liking it so much. The minor characters are fleshed out so well that they become really entertaining in their own right. A particular favourite of mine is the BOSS. To hear it humming along merrily is lovely;it is really heartening that the scriptwriters and actors take the time to give the extras real personalities of their own. Stevens is also a refreshing new take on the "unwitting-pawn-controlled-by-a-higher-intelligence" theme. To have a character who can both be completely dominated by the computer, and also remind him to get back on track when he is busy eulogising is refreshing, and his sacrifice at the end is moving: not least because the Doctor accepts that it is something that Stevens has to do to cleanse his conscience. His nod of thank you to Stevens brings out the best in Pertwee's Doctor.
The Doctor also feels somewhat rejuvenated in The Green Death. He is now leaping from cranes, running away from armed guards in sealed compounds, and scrambling through mineshafts. This "action man" feel to the Doctor helps to keep the story ticking along, and also makes him stand out from Professor Jones. If the Doctor was just a thinking Doctor, he would be the same as Professor Jones; but his extra energy gives him an edge over Professor Jones, as well as making his sadness at Jo's departure more profound, because of this juxtaposition.
So, over all, an excellent yarn. Complex characters and an all-action Doctor more than make up for some minor filming quibbles. First class.