One of the first episodes of pre-revival or "Classic Who" that I ever saw was the very end of "Horror at Fang Rock." This episode follows a rather basic premise. The Doctor and Leela end up missing their destination and end up at the Fang Rock lighthouse. The very same night, something falls from the sky and people begin mysteriously dying...and coming back. What follows is a classic murder mystery story with a supernatural twist.
Overall, this episode was a good first experience with "classic who" and is the serial I have shown people to introduce them to classic who. The simple premise, the straight forward monster, the cheesy and funny special effects, its short length, and its relatively brisk pace for classic who makes it a good way to get your feet wet. It also helps that there isn't anything racist in it that I can recall, which is very much an issue in classic who. As a matter of fact, the prior episode, "The Talons of Weng-Chiang" is possible the most racist episode of Doctor Who. Just to give an idea of what I mean.
The monster special effects and the lighthouse compositing are pretty bad. I actually didn't notice the compositing the first time I saw it, but my girlfriend pointed it out and now I can't unsee it. The monster, though? It's cabbage in a plastic wrap. It always makes me laugh.
Tom Baker is...listen, I can sit and watch this man read a phone book to me. The cadence of his voice is great in a way I have trouble describing. Leela, who has this kind of caveman thing going on, is a fun contrast. Their relationship does play into the "noble savage" archetype that is racist, but, as I said, this show is from the 1970s so you kind of take these things on board when watching. Louise Jameson does a great job portraying this strong, stern warrior woman who doesn't take any guff.
The supporting cast is overall strong and responsible. The lighthouse crew are just trying to do their job. You got the naive new guy, the old guy who seems tough but is secretly nice, and the guy in charge who wants to think to the future. Not too long after, a group of nobles join the mix. Their internal conflict over stock and honor end up being a responsible excuse for them causing problems that complicate the Doctor and Leela's plans to save everyone. Though, I do have to say the final death is the most confusing one of them all and a bit out of character. Why did THAT one go for the diamonds when THE ONE WHO DIED EARLIER seemed more the type?
Overall, it's a solid 7-8 out of 10 episode and a good way to introduce people to the show. It was my introduction and that worked.