Voted for Gogol, but also honourable mentions for Goncharov (obviously, though to be fair I've only read Oblomov), Lermontov (A Hero of Our Time was excellent, shame he died so young), Bulgakov & Solzhenitsyn.
Tolstoy is one I could never get my head round. I had to stop reading Anna Karenina after a few chapters (I very rarely don't finish a book I've started) because I found the main characters so unsympathetic, and it just generally boring. Maybe I'll pick it up again later; I'm trying a book of his short stories including Ivan Ilyich atm, but not overally impressed so far...
Surprised Kharms hasn't had more of a mention- as a taster, I particularly like the story about the man who was 'innoculated' against boulders continually falling on his head despite his brains seeping out because he already had a pebble in his eye
He get's a lot more surreal than that too..