Why such an emphasis on weight? If you're looking for the best size to weight ratio well then I think a little razor blade can't be beat.
Okay a serious answer, look to some of the Spydercos. They make some really light weight knives and their quality is top notch. Check out their Delica (smaller), Endura (larger), and Centofante 3 (middle ground) and see which one you like the most, as they are all very light weight.