A lot of friends that I respect and people on HN recommended "Master and margarita". I have only got back into reading ~4 years ago, so I haven't gone through all the great literature, but this felt different right after its midway point (starting with Satan's ball). My advice is to look for a translation with plenty of footnotes, because historical context plays a big part.
It's different to anything I've ever read, and definitely stays with you.