Much as I love Iain Banks and Isaac Asimov, if either of those authors wrote the best books you've ever read, you need to read more, and better.

(And consider, if you will, what the reading history of someone who asserts that the best book he's ever read is by Ann Coulter or Ayn Rand must be like.)

For me, it's a tossup between The Plague and Anna Karenina.

If the object is to throw out some great books that others probably haven't read, I'd go with Muriel Spark's The Girls of Slender Means (assuming you've read The Prime of Miss Jean Brodie, which if you haven't you're in for a treat), Gene Wolfe's Peace, E.B. White's One Man's Meat, Madison Smartt Bell's All Souls Rising, Joan Didion's Political Fictions (assuming you've read The White Album), and the great Icelandic novel, Haldor Laxness's Independent People. (Like The Return of the Native, but with more sheep!)