The bong in and of itself. the transition from natural wood (read: reactive) materials to the clean and pure (like a good christian child) material that is glass is the truest innovation to hit the smoking scene, ever.
im gonna have to go with the diffuser. It really does make a multitude of difference. If i was around for the advent of the GonG i would have to go with that, but I've known both GonG and grommet for my entire smoking career, so it takes some effect out of it