I doubt many people have more than two covers to compare (but on this board there may be one or two

I can think of three factors that would help- quality/thickness of the padding; size - larger would give more vertical cushion in a drop where the kindle is perpendicular to the ground; and how well the Kindle is secured within the case.
My M-edge for the K1 does a very good job in all aspects.
Any other thoughts?