Uhh, none. NEVER use a car cover. You can never get the surface in contact with your car perfectly clean nor can you EVER prevent it from moving, even slightly. Every time you remove it, you will cause scratches. Even the best cover will scratch your car.

Keep it clean, keep it in your garage and it will stay scratch free. Black car? a little dust is far better than lots of micro swirls and scratches caused by the cover.