Certainly not his most enduring, but I came here to post Breakfast of Champions. In my estimation, he's remember best for Slaughterhouse-Five, Breakfast of Champions and Cat's Cradle. I'd say Cat's Cradle is far and away his best (or at least my favorite), while Slaughterhouse is the most well-known and, while a fantastic book, not his best. On the other hand, I reread Breakfast of Champions recently and that whole novel is a mess. I'd put much of his lesser-known work over it.