The most unusual was Cloud Atlas, by Mitchell, so unusual that it isn't even easy to explain. There are six different narratives in it in from six different perspectives. They use six forms and are set in six time periods, starting in the 19th century and ending in a futuristic SF setting. Each ties into the following narrative, and each is split in the middle, so that after you read the last one to the end, you then read the conclusion of the earlier narrative until you're back where you started, on an obscure island in the South Pacific in the mid 19th century.
Second most unusual was Stapledon's Last and First Men, famous because the narrative arc stretches over billions of years. Stapledon's book is something of a landmark in the field and, although it is ambitious and unusual, I didn't find it particularly successful.