The Ambassadors of Death (Third Doctor).
An excellent story that probably could have been one or two episodes shorter. The case of mistaken identity regarding the alien ambassadors was well played and I liked that General Carrington's hostility towards them came from a mistaken impression during his Mars mission. However Reegan's plan to use the aliens in his theft schemes didn't seem well planned and the ending of the serial was completely anti-climactic. Carrington just gives up and the Doctor leaves.
Image of the Fendahl (Fourth Doctor).
A fun story but I had to think about this one a little as I was watching it as there seemed to be some large plot holes. There was no explanation about what the Fendahleen were, how the Doctor knew about them and why the skull was found on Earth. Then in the final episode the Doctor gave a good deal of exposition and the questions were answered. The one question that wasn't answered was why Stael thinks he can control the Fendahleen to give him ultimate power and how he found out about them in the first place.
The Invasion of Time (Fourth Doctor).
This one had a surprising start. The Doctor signs a contract with the Vardan to become absolute ruler of Gallifrey. It was fun seeing the Doctor act even stranger than usual. I'm not sure there was any real point to having the Vardan in the serial as the Sontarans would have worked just as well if they were used through the whole serial. I also liked how they showed more of the Tardis interior though the brick looked out of place. The relationship between the Doctor and Borusa is interesting and I hope it will be developed further. This was Leela's last serial and her decision to stay of Gallifrey didn't make any sense. Leela and Andred only had a few scenes together so the romance angle came out of left field entirely.