What 4chan boards would you consider high quality?
/tg/, with my obvious bias, though I also recall an old saying that describes /tg/ pretty well: it isn't about tabletop games, it's about tabletop gamers. A while back I would have said /a/, but it's lost much of whatever might have been redeemable and only kept the crap. /mu/ is generally decent, or at least accepting and prone to mocking le wrong generation types. I've heard that /s4s/ is friendly, but that's not personal knowledge and... 'tis a silly place (technically NSFW, too, duh). /co/ is pretty cool, especially since /mlp/ was split off and the argument cancer stopped. I've heard that /lgbt/ is accepting and generally chill if that's in your interests. /vg/ is the useful content from /v/ sans much of the bullshit.
But honestly, if you avoid the NSFW boards and /v/, pretty much any board is at least decent, as long as it matches your interests. /tg/'s great.
The main advantage, apart from topicality, is IMO that the nature of the board circumvents a
lot of traditional forum bullshit that was also the driving factor behind me coming to B12. No ego-stroking e-peen wanking; using a name means that (a) you're running a quest, external site, or other meaningful project or (b) an egocentric prick, at least as far as the community is concerned. There's no hugboxing or mod-friend shenanigans where you get banned or muted for disagreeing with a mod or someone they like. Content is enforced roughly evenly, in the sense that there are few rules but those are enforced strictly (there's still some inconsistency and bullshit, but far less than I've seen anywhere except bullshit).
Of course, people are generally harsher/more honest, and you're going to see a lot of fucked up shit in most of the NSFW boards. That said, I'd rather take the nastiness up front rather than in behind-the-back reporting, passive-aggressiveness, &c., but I understand why people don't.