These DH Shin/Knee pads from motorbike kit maker UFO offer plenty of protection, with a big hinged knee cap that pivots over your patella to allow easy pedaling movement.
The shin guards also extend lower than most for full flying rockprotection. The padding on the back breathes relatively well, and there are no bolts sticking through to cause you more damage than they save.
Our only major gripe is that although the outer shell pieces are seriously bulky - enough to clatter off top tubes and shifters regularly - the padded calf strap and above ankle strap are way too short. Even on fairly skinny legs we struggled to get much Velcro overlap unless we pulled them until they were too tight.
The level of protection that they offer also makes them somewhat on the heavy side, but the price is pretty reasonable.