The King's Demons is the worst Fifth Doctor story I have seen so far. Some of us non-English types bristle somewhat at the presumption that the Magna Carta is essential to World Civilisation; but even if it were, the Master's plot to derail it Makes No Sense. The Master is cunningly disguised for most of the first episode as a really bad English actor pretending to be a French knight. Turlough, who has always seemed pretty pointless to me, spends most of the story shackled rather unexcitingly to the wall. And we are introduced to Kamelion, the most useless companion of the whole of Who (and that includes Jo Grant, Dodo and Mel), which turns out to be the point of the story, if there is one. The best thing to be said for it is that it is only two episodes long.
There are several good things about Planet of Fire: we lose two of the more useless companions in the show's history (Kamelion and Turlough - and in fairness the latter gets one of his better stories here), and gain one of the better ones of the late period (Peri). The location filming in the Canaries and consequent swimming costumes are memorable. The problem is that the planet Sarn looks so much like the Canaries (for some strange reason) that it doesn't feel especially alien. Also the Doctor's behaviour at the end - where he euthanises Kamelion and apparently allows the Master to die in agony - is very un-Doctorish. Also the story itself is not all that interesting.
As a hormonal fifteen-year-old there was one thing and one thing only that I remembered about Terminus, but to my surprise it actually is a fairly good story as well. Tegan in particular has some good bits (and I don't mean her physical charms, though there is one moment when we cut from a view of her dramatically heaving cleavage to a shot up Nyssa's petticoat). The relationship between her and Turlough is nicely mirrored by that between the Doctor and Liza Goddard's Kari. Peter Benson's mad knight Bor is pleasantly reminiscent of Don Quixote. The plot doesn't make an awful lot of sense, but it is always watchable and sometimes compelling apart from two fairly huge flaws: the Turlough / Black Guardian relationship is now very silly indeed, and so is the Garm - not even trying very hard to look convincing.
Fandom seems to be generally fond of The Awakening; it didn't really grab me. Tegan's relatives have worse luck with alien invaders than those of any other companion pre-Rose. I found the Malus utterly unconvincing, and as so often its means and motivation made little sense. I did like Polly James as Jane though.
So, my verdict: Terminus definitely one to look out for, The King's Demons definitely one to avoid, the other two OK but not spectacular.