Die Welt von Gestern.
I've read very mixed reviews about it, saying it's mediocre compared to Ein Mann ohne Eigenschaften, but who has the lifetime to try to read that thing?
Me!
The Man Without Qualities is utterly fantastic and one of my favourite novels.
As for what I'm currently reading, I've been on a bit of a Chaucer binge: Re-reading the Parliament of Fowls (which is so lovely) and right now The Knight's Tale. He's probably underrated today due to the distance of his language (which I admittedly have some trouble scanning, particularly whether to pronounce the Es on the ends of words), which is a shame considering how fluent and musical it can be, and how witty, expansive and humane Chaucer is.