I'd certainly pick one of the Best Short Stories of the Year collections (maybe a Datlow), or maybe the McSweeney's Amazing Stories, or maybe a single-author collection like Ian McDonald's Cyberabad Days or Paul Park's When Lions Speak. But something about the way you asked this question makes me think you're looking for novels.
Hmm.
Well I'm already dead, but if it wouldn't weigh me down, I'd go with Neal Stephenson's Ananthem, because that thing takes up some serious time and is amazingly good. It has enough depth to really interest on a reread. And in a pinch, it can be used as a weapon.
If I wanted something a bit less brick-like, I'd go with The Last Witchfinder by James Morrow, which is also pretty dense intellectually speaking, but much shorter.