Or if we're not limited to just candy bars, then I also love the Wal-Mart brand (Sam's choice, maybe?) peanut butter cups. The peanut butter is totally smooth and doesn't have any of the grainy texture that other peanut butter does; apparently, when they make 'em, they inject it into the cups. They're fantastic.
ITA! What a great find that was! I used to be able to find Peter Pan brand pb cups in the miniatures around holidays, but I haven't seen them in a while.
My favorite is 100 Grand or just a plain ole Hershey bar.