UN LUN DUN by China Mieville. I'll think about more nominations later, but I skimmed the list and didn't see it, and I had to put it out there. It is a bracingly subversive tonic against the hackneyed "classic" quest narrative with the pretty white Chosen One that's so ubiquitous in YA fantasy especially, in many ways.

Also Stand on Zanzibar and The Sheep Look Up by John Brunner (for greater expediency these can be grouped with Shockwave Rider as the Innis Mode novels, but I personally didn't like Shockwave Rider that much), novels exploring ideas and worlds through mosaic story-telling techniques incorporating excerpts from other texts, ad copy, jokes, scraps of conversation, and so on with more conventional narrative elements and using large casts of variously situated characters to build an immersive encounter with a realistically effed-up world.