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I think /lit/, for a couple of reasons.
All of 4chan has poor quality posts, but /lit/ is one of the few places here that encourages and engenders longer discourse and argumentation. They have a longer post limit. /sci/ posts tend to be more vague, shorter, and inarticulate on average, even if they have some good resources.
/lit/ has a large portion of users who aren't broadly in tune with all of philosophy/literature necessarily, but know a lot about a given topic or author to the level of an expert. There's people who are really familiar with Plato, Kant, or Hemingway, and you can actually learn something from them.
/lit/, at least right now, has far more self educating users with no formal education in whatever subject. /sci/ has far more niggers who only care about uni degrees or just read popsci these days. And many /lit/ users are actually majored in mathematics.
But out of all boards directed towards some intellectual endeavor, /his/ is by far the most retarded. Far more than /lit/, /sci/, or /g/.