Another fan of the Schwalbe Marathon Plus here. I've never had a flat with them. They're not the smoothest riding tire but they seem to be nearly flat-proof. They were a little difficult to mount on the rims but it's really not an issue since I probably won't have to remove them until they're worn-out. If you're willing to sacrifice some ride quality (and they don't ride that bad) for the assurance that you're not going to flat, these are the way to go in my opinion. I've also run Specialized Armadillos and Panaracer Pasela TourGuards but I've flatted with both of them.