The best one, I think, in terms of not adding bulk but still providing protection, is the Belkin. . . .but it doesn't appear to be available any more.
The stock cover, which comes with or without a light, is nice in that it is very light. . .good rubber bumper holds the thing in tight. But there's no real closure and I find the leather a bit slick.
The Marware is also nice; comes in a variety of colors and styles, and has a nice feel. Most of the styles do not add much weight. But they're generally designed to fit
either the basic Kindle or the Touch; with the basic Kindle you're left with a half inch or so around the device which may or may not be annoying to you. I, personally, like it to fit pretty much exactly.

JIVO is similar. From the descriptions, there might be a similar issue with MEdge products.