I've heard great things about the Conti Four Seasons tires but have never ridden them. My swear-by tires are Panaracer T-Servs, which are burly enough that I would be pretty secure riding them on gravel and have the best traction of any city tire I've ever used. They might not be the fastest "slick" out there, but then, they're definitely faster than purpose-built cross tires on pavement.