I won't take back my VSOF comment at this point, but I agree that Blanton's is pretty dang smooth, too. One exception - a little while back, Tina and I were at the Homestead, a hotel/resort in western VA. I went to the bar to get a drink, and Blanton's stood out, even though the choices were really fairly solid. I got a Blanton's on the rocks for us to share (thank goodness) and just about fainted when the bartender said, "That'll be $15.86." Took a little of the smoothness away. Still, we made sure we drained the glass, and I ate some of the ice, too.
The smoothest bourbon I can think of, off the top of my head, is Blanton's.