r/AskHistorians - Just what it says, but it is probably one of the best subs on the entire platform in my opinion. And I'm not even a history buff of any kind. It's just super well moderated and has clear rules that get to the heart of the purpose of the sub.

r/fantasy - I read a lot of fantasy novels. This is probably my most visited sub on all of reddit. It's great. They get a lot of fantasy authors to participate and just generally have good discussions. That said, I feel they may be having some growing pains lately as I've found a whole lot of combative discussion lately on some of the more popular authors (like Brandon Sanderson), where someone says they love that author in one post, then almost within the same day a hate post comes up that tries to counter the positive post. It's.... annoying and I sure hope the mods can figure out a good method for stopping this shit as it's super obnoxious. (People, you're free to your opinions, you don't have to tell the world if your opinion is different than someone else's. It's not a zero sum game.)

There's a lot of really good subs out there, some more useful than others, some run by idiots, some have far more effort put into them than there should be.

Edit; formatting, spelling