I've watched a few more Classic serials since last posting so thought I'd continue writing up short reviews. SPOILERS ahead.
The Planet Of The Spiders - Unfortunately Jon Pertwee's regeneration serial is a stinker, certainly the worst of Classic Who I've watched since it was put on iPlayer. Who producers often boast about their stories being more ambitious than their budgets but here the lack of money, or its misallocation, really drags the story down. Every scene on Metebelis looks dreadful because of how the actors are very conspicuously layered onto green-screen images, which is often the case for the spiders as well. The second episode is one long chase using various ludicrous vehicles, which is sort of amusing for a little while but brings nothing to the story and makes you wonder why the money spent on that wasn't put towards making the other episodes look better. Outside the main cast, the acting is pretty lousy at points (especially among the Metebelis humans, though a little credit to Gareth Hunt for trying to treat this ludicrous material seriously) and it didn't seem immediately clear to me what killed the Doctor - radiation of some kind, but it didn't seem to affect him much up until falling out of the TARDIS back on Earth - or what the fear he supposedly had to face was. Admittedly I might have just been too bored to notice the key details by that point, which is damning enough in and of itself. Rimpoche and Choji being (the same) Timelord should be more interesting than it is - is this the only story with two regenerations in one episode? - but feels completely tacked on. The potential was there for this to be pretty good with a modern budget (and better storytelling), but unfortunately it's a bit of stinker on a shoestring.
The Ark In Space - Ark is Tom Baker's second serial and while he hasn't quite settled into the role yet, he's already a very charismatic and compelling lead. The base-under-siege story is Who staple and this is a strong early example which paces its story well across its four episodes. The Wirrn aren't particularly interesting foes by themselves but having Noah be gradually taken over by them, until becoming their figurehead, works well. Although his infected hand at the end of the first episode looks a bit too silly, very obviously being coloured bubble wrap, the second stage of his infection, taking over most of his body, is very creepily done and the grub crawling across the space station floor also looks great (both are also obviously bubble wrap, but actually look 'designed' rather than just taped to the actor's hand). Other than Harry being a bit annoying as a companion, I haven't any serious complaints though I also don't think the serial had the magic spark to elevate it to the top tier, settling for a perfectly respectable 'very good'.
The Brain Of Morbius - I'd read that this one was considered a classic and it's certainly dense with lore and very enjoyable for three of the four episodes. Philip Madoc is tremendous as Solon, Tom Baker has completely settled into his groove as the Doctor and the makeup on the Sisterhood gives them a witchy vibe which plays well against the Gothic horror in Solon's mansion. Despite the story mostly consisting of the Doctor and/or Sarah going back and forth between two fixed locations, it's good stuff until it falls to pieces in the final episode. The Doctor tries to murder the villains by gassing them, leading to a completely underwhelming death for Solon, followed by another (pushed off a cliff by the Sisterhood, whom he should be able to slaughter easily) for Morbius. The infamous contest of minds between the Doctor and Morbius is completely pointless, Condo falls in love with Sarah completely out of the blue, the Doctor completely trusts Solon to destroy Morbius against all logic... it's such a mess that it ruins much of the good faith built up by the three preceding episodes.
The Time Meddler - Absolutely superb, with no caveats. The story is not only extremely well-paced - four episodes and no more really is the sweet spot for Classic Who serials - but the characters are engaging, the performances are uniformly strong, the mysteries remain intriguing throughout, and the cliffhanger of the third episode is an absolute barnstormer, maybe the best of any Classic Who episode I've seen. I love Hartnell's Doctor and he's on great form here, bouncing between mischievious and irascible as best suits him, and Peter Butterfield's softly comedic performance as the Monk provides him an outstanding foil and the series a more interesting villain than the usual world-conquerors. The Monk is like the Doctor without any sense of responsibility, playing with history for his own amusement (his initial claims of wanting to 'do good' don't last for long, and his joke about assisting in the building of Stonehenge is great). Steven is a perfectly serviceable replacement for Ian, a bit generic perhaps but likeable and with some nice touches in his writing (being from the distant future, 1066 is so far in his past that he's unsure whether Saxons used artillery or not), and while Vicky isn't quite up to Barbara's level, she's a big improvement on Susan's non-stop screaming and whining and her willingness to take charge of situations, while also being sweet and funny, makes it immediately clear why she deserves her place in the TARDIS. I'll rewatch The Aztecs at some point, which I remember being my favourite First Doctor story, but The Time Meddler will be extremely strong competition and of all the Classic Who serials I've watched recently for the first time, this is the best one (I'd seen Remembrance Of The Daleks before, so am not counting that one, which remains my all-time favourite classic serial). Top of the line stuff.