Beside passing light through vacuum--which would be an "obvious" solution--there are a few ways to create "super transmisivity", i.e. no optical attenuation when passing through point A to B.
1) Propagating light below the bandgap of a material: According to Kramers-Kronig relationship, index of refraction of a medium is 1 when a radiation field propagates far away from the atomic resonances present in the medium. This is more or less of a passive process, i.e. you cannot control the window of transparency, beside selecting different medium depending on the wavelength of the light.
2) Electromagnetically induced transparency (EIT): This is a very well understood and common experimental quantum optics phenomena, where through a use of pump beam one can prepare atoms in a very specific state such that the probe beam passes through the pump without any attenuation. If you were to switch the pump beam, the probe beam gets scattered by the atoms making the medium opaque. With EIT not only you can control the transparency window (frequency) of the medium but also make the medium opaque or transparent "at will"--some sort of an optical switch. In addition this process gives rise to an interesting phenomena called slow light, where one can slow down the speed of light. I am not sure (and don't remember the details) if people have achieve a unit transmission of the probe beam making it truly transparent.
PS: The system does not let me comment (as I don't have enough reputation) thus typing my comment as an answer.