As mentioned, lots of factors, are there any loose surfaces, gravel, sand, etc. where you would want some tread, or is it all hardpack? What's your size, and offroad handling skills? Lots of factors to determine tire size and type. But typically, a slick will be fastest for road/smooth hardpack. If you need some tread, then Continental Terra Speed or Schwalbe G-One Allround are the fastest knobbed tires.