First, the search feature on every single forum ever is absolutely awful. When a topic fades from being current on any board, you should assume it's gone forever. Google might help, but it probably won't. The Internet is a big black hole.
ENWorld -- If I were to describe it to someone who hasn't used it, I'd say it's a place where DMs and homebrewers talk about things. Moderation is relatively good, but like a lot of boards tends to allow topics to go just a bit longer than they should. Has too many posters who confuse discussion with arguments so they just argue in circles, resulting in every thread being functionally worthless after ~100 posts because a few posters arguing about an often tangential and irrelevant point monopolize the thread activity. Recommended, but you should avoid long threads unless you like to smash your face into a wall. Also good for hobby-wide or industry-wide news.
GitP -- There's a strong character optimization monoculture left over from a large 3.5 charop and 4e charop community, but if you can accept that then they can be good. I really enjoyed my time here during the D&D Next playtest, but I left for about a year and everybody I knew seems to be gone after the boards were reorganized to merge 5e stuff in. There was a lot of edition warring, but I think we were all still working through why 4e did or didn't work for our tables. At one point they had some amazing in-character boards and play-by-post stuff, but I haven't been there in a very long time and I would be surprised if it's still that good or that popular since the rise of VTTs.
RPG.net -- I've had really good and really poor experiences here. Moderation is hit and miss to extremes, IMX, but different topic subboards feel very different. I avoid it because I feel like I'm just going to get in trouble. I feel like I'm supposed to agree with the politics, and even though I think I generally do, I don't feel welcome there because I feel like I'm being policed? That's all really. The site makes me feel unwelcome so I don't go back.
D&D Beyond Forums -- No real culture here that I can tell. Almost everyone posting is a newbie asking pretty basic questions. None of the boards that interested me had any interesting topics. Moderation seemed fine, but it's hard not to be when the volume is that low.
RPG StackExchange -- Highly strict moderation; worse than most Stack Overflow sites, IMX, and I've got about 24k on StackOverflow itself so I use other SE sites a lot. This site practices dogmatic worship of RAW. Want to post? Better have chapter and verse at hand. A very significant number of posts seem to be rules lawyer-y in nature. Most other questions are some form of, "Can you read the rulebook back to me?" which just doesn't feel useful or fun. Useful when you want a strict RAW answer, but otherwise it should be avoided. Not very welcoming.
Twitter -- Worthless for discussion. Nice that Crawford sometimes responds. Much worse now that Mearls doesn't post, as I felt like the two together are what made 5e work well. Crawford is very much just interested in reading the book back to you. I understand why that is, but it's not useful or interesting to me because I'm going to take that same information and do what I want anyways.
Reddit -- There's several boards, but in general it's pretty good if you can put up with everything that sucks about Reddit. The fact that topics fall off the top so fast is great for new topics, but it makes long form discussion basically impossible. ENWorld makes long form discussion too easy; Reddit makes it too difficult. Every subreddit has it's own culture, some good and some bad. There's a ton
of often needless posting just for karma, especially image posts, and while it's sometimes cool it gets old fast. The bigger the sub, the more low-effort image posts there are. There's also a lot of groupthink, and the upvote/downvote system encourages toeing the line.
- /r/dndnext, /r/dnd -- I find these basically indistinguishable. A lot of new players making the same mistakes that all new players make. A lot of intermediate players making the same mistakes that all intermediate players make. A lot of D&D streaming fans who mistake their favorite streamer's style as the One True Style. Way, way too much art, maps, commissions, photographs, etc. Sure, it's cool that people do that, but I don't want that to be 90% of the content and it kind of is. I don't need performative social media hobbying. A lot of players and DMs who are less experienced than I am and are stuck in modes of thinking that I've moved past.
- /r/DungeonsAndDragons -- Crossposts of all the image posts from /r/dnd and /r/dndnext. Skip it.
- /r/rpg -- A more diverse community than /r/dndnext and /r/dnd. Still a lot of art posts. Some posters are anti-D&D because D&D is D&D, probably from players who have finally tried something other than D&D. The good posts are a little sparse, but they're definitely there. Worth checking out. Along with ENWorld is good for hobby-wide or industry-wide news.
- /r/DMAcademy -- This has the most interesting topics, IMX, but it's a bit small. Some homebrew content that I find useless, but overall pretty good. Recommended.
- /r/DnDBehindTheScreen -- Not quite as good as /r/DMAcademy, but decent.
- /r/mattcoleville -- Like a mix of /r/DMAcademy and /r/rpg. The good parts are really good. Recommended.
- /r/UnearthedArcana -- A good resource, but most topics are "look at this thing I made" which doesn't make for great discussion. Community seems great, but it's not offering something I'm looking for.
- /r/Homebrewery -- Very useful resource for the The Homebrewery - NaturalCrit site, which is great for making your homebrew look like 5e.
- /r/AdventureLookup -- Useful resource for the https://www.adventurelookup.com/adventures/ site. Pretty dead now that the site works well, but at one time it was really useful.
- /r/3d6 -- A charop board. Surprisingly low volume, and the posts aren't very interesting. A seemingly endless number of "How do I make a Sorlock with these stats" posts. Not enough volume to maintain my interest.
- /r/adnd -- For people actually running 1e/2e stuff, without as much politicizing as other boards. Good stuff if it's what you're doing.
- /r/osr -- Eh. Personally I found sitting down and reading through the Principia Apocrypha more enlightening and interesting than anything I've read here. Seems to be a lot of posts that are about why OSR is good, and not so much on just playing the game. Someone else said it, but it definitely feels like people are a little elitist.
Min/Max Boards -- I remember these being great like back in the 3e era, but looking just now they're completely dead. RIP.
I've never gotten in to any Discords or Discourses. Just hasn't interested me enough, so I can't comment on them at all.