The Key to Time: The Power of Kroll (Fourth Doctor).
There's a good story in here but unfortunately it seems like Robert Holmes wasn't able to pull it off as he had in the past. The characters spend too much time going in circles and their motivations aren't flushed out. The audience is given no information as to why Thrawn wants to kill the Swampies, only that methane/protein project is his baby and his desire for no interferance has made him evil. Philip Madoc is also wasted in his role as Fenner. He probably would have done a better job with Thrawn. It was also good to see John Leeson get a small role since K-9 was stuck in the Tardis.
The Key to Time: The Armageddon Factor (Fourth Doctor).
The last serial of The Key to Time storyline and probably my favorite of the lot. The story was complicated, but not overly so, the characters were well written and played and the comedy worked well. My only gripes are that Princess Astra's role kept getting smaller though she is one of the most important characters of the serial and the audience is given no further information about who the White and Black Guardians are. The Doctor finds the key for the White Guardian, breaks it before the Black Guardian can get it and that's it.
It's too bad that Mary Tamm decided to leave Doctor Who after only one season. Her version of Romana is, in my opinion, the best companion the Fourth Doctor has had yet. However it is entirely possible that Lalla Ward's Romana will make me change my mind.