If you occasionally ride off-road, take a look at the Continental Town & Country, or the equivalent Serfas Drifters. They have an inverted tread design (recesses instead of knobs,) with a continuous slick center portion on the crown of the tire around the circumference, which provides a nice, smooth portion for the friction roller contact. They ride very nicely on the pavement, and only under hard cornering do you ever feel feel the tread.