The Master and Margarita by Mikhail Bulgakov.

What can I say that hasn't been said here already? His humor stands out. For example, when characters are in fits of confusion and subsequent hysteria because some impossible thing just occurred, they'll throw up their arms in surrender and shout "Ah! The devil only knows what is happening here!" And it turns out that that is exactly so; the Devil indeed teleported him to a far away city or hypnotized him and had him sign a major theater contract, etc.

The prose is stunning (Burgin and O'Connor translation). The depth of his description is hypnotizing. I don't have it with me, otherwise I'd open to a random page and paste any paragraph as evidence.

Finally, the historical context in which the book was written makes it all the more fascinating. From my limited understanding, Bulgakov was walking a tightrope in his dealings with Stalin. It is well established that artists around Bulgakov could simply vanish as a result of having crossed some political line. Nonetheless, Stalin appeared to have deeply admired his work, while still suppressing a few plays here and there. With annotations I was able to clearly see Bulgakov's subtle jabs at Stalin and the Soviet state. Not to mention the premise of the book itself, which stands in stark contrast with the atheistic government under which he lived. He is one of the most efficient writers I've ever read, saying more with a few sentences than many could achieve on a page.

The only negative for me was having a hard time with the names: first name, patronymic, last, all used at different times throughout. Thus you end up having to memorize three names for each character. I was often confused. I know this is just an element of the Russian language and therefore it's fiction, but I found it more difficult here than in other Russian works.