probably the haro prolite, I'm warning you they are about 800 to 900 bucks Canadian
it depends on what sort of rider you are going to be, if your going to be a skatepark rider (tricks ect,) go to your nearest bike store and look at different ones in your price range.
The lightest bmx bike is owned by Samuel Powell. His bike ways 18.3 pounds and is built custom from the ground up. It totalled to about $2,300.
The lightest bike for adults is probably a redline or a hyper but hypers are kind of expensive
4 lbs. 1 oz.