Some of the best are likely the OX steel frames from Standard Industries which have significant strength increase without added weight. But, they aren't really the lightest frames... Probably the strongest though.
Why do people care so much about weight? I mean, you can cut some corners, but your FRAME only weighs between 5 and 7 pounds usually. The other stuff makes up another TWENTY FIVE pounds! That's where you can cut a lot of weight. Just make sure your frame is really good and your fork is really good, then start looking elsewhere for some weight savings.
i used to ride a kink empire revision b.. that was pretty heavy, but yea totally.. It is much better to reduce other ways. Some people are rediculous abou tweight though, especially around here.. like people using plastic barends instead of aluminum for "less weight." they could just empty the change from thei pockets for the same weight reduction.