Boy... narrowing it down to a candy BAR is tough. Candy in general is much easier. As much as I hate to admit it, I absolutely LOVE candy.
As far as candy bars, I would have to say a Butterfinger or Caramello really hit the spot. The new Oreo candy bar's are quite good as well. Reese's cups are always good, though not a "bar" in the strictest sense.
Beyond that, I'm a sucker for gummy bears/worms, rock candy, Nerds (and it's delicious cousin, the Nerds rope), and heck even chocolate covered prezels!