Most transparent material

[Fixing]

what is the most transparent material for visible light?

[minor]

I would say Quartz which is 99.99% Silicon Dioxide.

[Powerlab]

 I think vacuum is the most transparent 'material', or when there is no material at all.

[Retrotech]

For any light (visible or invisible), I think the material will depend on the wavelength (or at least the wavelength around which the light is centered). If you are dealing with a very narrow wavlength range (such as a laser), then you will have to base your choice on that specific wavelength. Losses through a material can also be minimized by using the Brewsters angle at specific wavelengths.

http://en.wikipedia.org/wiki/Brewsters_angle