If you are a video game enthusiast, particularly of a niche genre, /vg/ can be a good place. I'm really into roguelikes, so I visit the '/rlg/' thread around once a week to see what's going on and potentially get some advice if I'm at a tricky part in a run or need to make a decision about where to take my character.
Standard disclaimer applies about 4chan. If you're thin-skinned, you may find it not worth the trouble. Just don't engage with the trolls and you'll be fine.
/tg/ is truly excellent for non-video game discussion (board games, card games, w40k, MtG, etc). They are a bit opinionated much like /vg/ and I'd agree, thin skinned people will be happiest if they stay away. For example there is little love for Paizo/Pathfinder on /tg/ (although there are some fans...)
I spend a lot of time on /out/ as in outdoor hobbies because I live in a "recreational state". No online community is 100% perfect but its bizarre how /out/ is on average more civilized than heavily moderated communities.
I don't know what it's like now, but for a time /m/ was a major hub of the tokusatsu community, and most of the major fansubbing groups got their start on /m/.
/diy/ is really great. Generally post quality is inversely proportional to board popularity. Also avoid any board related to entertainment if you want rational discussion.
To provide a concrete example of why /diy/ is great, most "diy-ish" discussion boards I participate in devolve into "look how awesome I am" "look how much money/debt I can spend" "I am more of an authority than you" "my favorite authority could kick the butt of your favorite authority" and similar self-aggrandizement-posting. People on /diy/ actually talk about /diy/ topics, which is a refreshing change from most non-anon non-chan discussion boards. Its a relatively low-sophistry zone... emphasis on relatively...
The biggest problem I see on /diy/ is questionable humor, sort of like how blue collar occupations used to haze the apprentices a bit, you'll see the most ridiculous / comical suggestions sometimes.
Standard disclaimer applies about 4chan. If you're thin-skinned, you may find it not worth the trouble. Just don't engage with the trolls and you'll be fine.