I wonder if you'll find it reassuring or chilling that tintin++ is still under active development and is arguably still the state of the art for mud scripting.
I had to reverse engineer a telnet interface for work last year and I did nearly all of the exploratory work in tt++. Got me back into mudding after many years away though. Once I had mastered that tool, why not.
I like discworld mud, it's loosely based on terry pratchett's books but has drifted a lot over the decades it's been active and is largely its own thing now.
Achaea is still really popular and active but I didn't spend all that much time on it when I was getting back into muds. It's got some freemium aspects to its model that turned me off but I understand is really well-liked by its players so probably worth a try anyway.
I also hear good things about alter aeon and it seems to be really active. I've been meaning to check it out but haven't yet.
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A quick look makes mudding look more popular than it really is. There are hundreds of unique operating muds, but probably fewer than a dozen that ever see more than 50 simultaneous players. You could pretty easily narrow it down based on that and choose a couple whose theme appeals to you most. But also if you get into it you're choosing a community as much as a game, so observe and feel out the culture a bit before you get too invested.
And IMO skip anything that allows concurrent multiplay. Those look more busy than they are; in reality it's going to be a handful of people who have known each other for years each multiplaying dozens of characters.
