While there is no “consensus” as to what the greatest novel(s) of the 20th century might be, almost all the leading contenders are to be found on this list:
Two non-English contenders are The Master and Margarita (I see already mentioned upthread) and If on a winter’s night a traveller (Se una notte d’inverno un viaggiatore) by Italo Calvino.
Another great Italian is Primo Levy: The Periodic Table.
…and, for the most part, who have penises. I counted six women on the list, with no Alice Walker, Maya Angelou, Doris Lessing, Barbara Kingsolver, Toni Morrison, Zora Neale Hurston, Margaret Atwood, Isabel Allende, etc., etc.
Maybe not all of them deserve to be on the list, but I would expect to find at least a few of them.
Since these are the greatest works ‘in our opinion’, I have to go with the titles that were the most meaningful to me personally.
Brave New World - Huxley - incredibly thought-provoking
LOTR trilogy - Tolkien - the sheer beauty of language and the power of mythology
The Once and Future King - T H White - taught me that a non-fiction master writer could put out a darn good novel, too
Centennial - Michener - inspired my lifelong love affair with the American West and taught me the value in impeccable research, even for a work of fiction