Schwalbe Marathon Dureme in 700 x 40. A little more toward the touring/road side but they handle dirt trails and gravel just fine (desert around here, so no mud to report). Superior to any other tire I've tried (which admittedly isn't many) in every way except cost -- lighter, faster, tougher, smoother ride, grippier, easier to mount. No flats yet and it's goathead season! I am completely sold on these tires, you really do get what you pay for. Schwalbe pulled out all the stops designing these.