Well, I'm still trucking through the Tom Baker years.
The Robots of Death
I liked this one, it maintained interest throughout. Some excellent model work in this one, I really liked the sandminer and the rocks falling as it moved along. Other nice work included the Doctor being buried alive which looked quite real.
Having the miners be a decadent posh lot made for a change of pace from the usual. The costuming, makeup and decor used in conveying this concept added to the visual punch. The robots themselves, being both possessed and not, and the different models added a lot of variety and ambiguity to keep things interesting.
I think this one passes the Bechel test as well.
EDIT: And with the red eyes and all this must have been the inspiration for the Ood.
The Talons of Weng-Chiang
Well-known and oft cited as a classic episode this uses the BBC's prowess with Victorian settings to good effect. (It also shows Doctor Who's struggles with visual FX with the mutant rats).
It's a bit eyebrow-raising to watch these days with having a white man in Chinese makeup and the unfortunate "most..hon-or-a-ble" stereotypical accent and inscrutability (or is that in-scroot-a-bill-a-tee?). That said, it's a pretty good make-up job and you get used to it soon enough.
Lots of good side characters in this one with the famous Jago and Litefoot and the creepy Mr. Sin.
I've noticed there's a lot of defenestrations in this era and this time Leela gets to dive through the glass. Leela works well throughout contrasting her "savage ways" with Victorian manners. Once again, she drives a lot of the action with a lot of independent action.
Horror of Fang Rock
Doctor Who is many things and this is probably the most primal distillation of the base under siege story. A bunch of people trapped in a small space get picked off one by one by a monster. And that's about it, I could see where one might like the pure and singular focus of this element of Who but I needed more to keep me engaged.
My favorite moment is that shot of The Doctor hanging off the window of the lighthouse. Something about the dangling seen from a medium distance was quite unusual. Not quite unlike that infamous umbrella cliffhanger from Dragonfire.
The Invisible Enemy
This serial could have been better served if it had better visual FX. The story isn't all that great but the spaceships, laser battles, and the Fantastic Voyage of the Doctor and Leela and the macrovirus could have distracted from it had they been better executed.
Probably at the time the debut of K-9 would have been quite a fascinating wonder. I would've loved that as a child no doubt. I didn't remember that funky "tongue", I can see why they dropped (or retracted?) that. It's too bad his weapon is particularly badly rendered further killing the potential visual dazzle of this story.
There's a great moment where the Doctor runs into the TARDIS and his scarf gets caught in the doors which then gets a quick tug in before the TARDIS disappears. Probably unintentional but it's a great "Indy grabs his hat" kind of moment.