For me it's the German ones. I moved to Australia in the late 90s, and they help me keep up with things a tiny bit, so the culture shock isn't too big on my rare outings back home.

Also cats. Cats are nice.

I'm a bug enthusiast, so insect specific subreddits like r/moth r/weevil r/entomology

Game ones for all my random niche games I like.

But also I have a lot of health problems so going to those specific subs with my specific problem and knowing I'm not crazy felt really liberating. I know there's a new disability thing on here, but collectively being in a community with the people who have my specific problem made me feel less alone.

All of them. There’s so much collective knowledge stored in the comments, posts and subreddit menus, and it’s going to disappear. Stupid spez

r/unresolvedmysteries

/woodworking folks posting about the stuffy the build out of wood.

I'm going to miss r/bushcraft, r/hammocks, r/ebikes, and even r/worldpolitics lol

/r/watches

A bunch.

r/guineapigs - it was very helpful for new pig owners, and a great guinea pig fix when you're at work.

r/disney* - for a Disney parks nutcase like myself, the various Disney park related subreddits helped me get over the time between visits.

r/bestofredditorupdates - I get frustrated reading posts and not having any idea of how things ended up, but boru allowed me to indulge my interest in others' stories without getting frustrated by the lack of closure.

The list goes on and on.

R/crt and r/crtgaming are the first that come to mind. Damn i love old tube tvs.

At the moment, /r/horrorlit is one of the subs I browse the most and I haven't found a specific replacement for.

/r/skeptic

the official UTDR subreddits. found some of them here but theyre pretty small (like 20 members or smth)

/r/hoggit and some flight simulation ones. Someone created an instance to cover this but seems it went down. I guess this madness needs some settling down.
Eventually I'll end up finding my way.