Destiny of the Daleks, first broadcast in September 1979, was Douglas Adams' first story as script editor and Lalla Ward's first as Romana II, the regeneration happening for reasons utterly unexplained (until the second series of Gallifrey audios). The Tom/Lalla sparking is great fun; sadly the same can't be said about the rest of it. I hate anthropomorphic robots, and the Movellans certainly qualify; David Gooderson is really not much cop as Davros; and the Daleks are particularly pointless. Alan Stevens and Fiona Moore have defended the story, but I think their hearts are not in it.
Revenge of the Cybermen has a rather poor reputation among fandom, but I rather enjoyed it this time round (I remember it first time round in 1975, when the second episode was broadcast on my eighth birthday, and watched it again in around 1990). If you treat it as Doctor Who and the Vogans, rather than pay attention to the irritating Cybermen, it is a great story - the three main Vogan characters, played by well-established pillars of Who like Michael Wisher (Davros), Kevin Stoney (Mavic Chen/Tobias Vaughn) and Ian Collings (later Poul in The Robots of Death and Mawdryn) all spark off each other well and give a convincing picture of a paranoid, factionalised society. Unfortunately the Cybermen rather spoil the plot - they have a different crucial weakness each time they appear, it seems - and I can never watch the Doctor's discovery of the Plot Device Cupboard in the last scene without wincing.
Terror of the Zygons is a real classic, also from 1975. I went for it after reading both the follow-up Sting of the Zygons and re-reading the novelisation, Doctor Who and the Loch Ness Monster. While the novel is slightly better - the TV Skarasen looks really crap, the TV Scotland looks peculiarly like West Sussex and the Doctor's banter is slightly funnier in the book - the original version is still pretty good. One thing it has that the book lacks is fantastic incidental music by Geoffrey Burgon, who went on to great things (incidental music for Life of Brian, all the music for Tinker, Tailor, Soldier, Spy and Brideshead Revisited, incidental music for Silent Witness). And the Zygons themselves, as my Australian friend said so many years ago, remain "two-cushion monsters". Plus it's a decent final story for the Brigadier in his military role.
So, two good, one so-so.