Planet of Evil, which followed straight on from Terror of the Zygons in 1975, is not bad, though not quite great. Frederick Jaeger as tortured scientist Sorenson is great, even if it's a scenario that we've seen before. There are hints that Morestra is a real planet with a real political culture - how else could the commander, so out of his depth, have been put in charge? The Sarah/Doctor relationship is good too, and we have hints of the Doctor's concept of his own role in the universe. This is in principle a similar story to a couple of Pertwee ones, Colony In Space and The Mutants, but ends up being nothing like as awful. But somehow it doesn't quite come together to be memorable; if you've see the video collection "The Baker Years", where Tom Baker reminisces about his time on the programme, this is the one story he can't even remember making, and I can see why.
The Invasion of Time, which was the 1978 end-of-season six-parter, is nothing like as bad as some people say. The Doctor's apparent treachery to both Leela and his own people is very puzzling and watchable, and the scene when he explains to Borusa what is really going on is great; the cliff-hanger at the end of episode 4 is one of the best in the show's history; not one of Leela's great stories, especially given the unconvincing romantic departure, but Louise Jameson is as always very watchable; the concept of the Tardis having a vast hinterland of rooms to explore jars with some fans but not with me (in New Who, I wonder what lies beyond the console room of the redesigned ship). K9 also gets a reasonable amount to do, though the noise from his motor is painfully loud. The Vardans are a bit disappointing, but hey, who cares if one of them seems to have a Scottish accent - don't lots of planets have a Scotland?
The Horns of Nimon was a story I missed first time round, as we were abroad for the Christmas season of 1979-80, and I realised it was the first story I'd seen with David Brierley doing the voice of K9 - rather jarring for those of us who are used to John Leeson's chirpy tones. Again, this story has its detractors; I felt it was not awful, though again not really very good - the Anethans are so wet it is difficult to see why we should get excited about their survival; the Nimon costumes are pretty poor; the whole reading across from the Theseus myth feels contrived in a way that, say, Androids of Tara/Prisoner of Zenda and Brain of Morbius/Frankenstein don't. But Graham Crowden as Soldeed is superb, and the Doctor/Romana relationship is not quite as obviously romantic as in City of Death but still very watchable.
I did catch Black Orchid first time round in 1992, and it was and is a rather charming story, the Doctor and friends relaxing in a 1920s country household and uncovering the family secret. Davison, playing cricket, and the two girls, partying and in Sutton's case playing two roles, are great; Waterhouse as Adric won't dance but will eat. I am left a bit uncomfortable, however, with the idea that you should hide your disabled relatives upstairs and then let then fall to their death off the roof.
From time to time as I've been watching old Who - say with The Sensorites, The Dominators, The Mutants, The Ultimate Foe or Battlefield - the thought has occasionally crossed my mind, "This story is bad; but is it as bad as The Twin Dilemma?" Memory fades, and sometimes fades mercifully; I remember thinking the The Twin Dilemma was bad on first broadcast in 1984, and rewatching it totally confirms that view. Every time I began to think that perhaps it wasn't as bad as one of the other low points of Classic Who, it suddenly got worse again. Where to start? The extraordinary alienating and inconsistent characterisation of the Doctor; the utterly lousy acting of the titular twins; the appalling costumes of the gastropods; the basic incoherence of the plot (to pick two aspects of this, the implausible and poorly developed Doctor/Azmael relationship, and the bizarre decision by the Doctor and Peri to leave Lang alone and armed inside the Tardis); oh, it's just awful. It's amazing that the show lasted another five years after this.
So, in summary, Black Orchid and The Invasion of Time are surprisingly watchable despite their flaws; Planet of Evil not quite as convincing; The Horns of Nimon decidedly less so; and The Twin Dilemma should be skipped. It is firmly at the bottom of the Dynamic Rankings site and likely to stay there.