When wave propagate from one media to another media,
i.e. wave has different index of refraction (or wave speed is different in two different media),
then some wave will be reflected.
The closer the index of refraction, the less amount of wave will be reflected under the same conditions.
If you want more wave to be reflected, you need to find larger differences in index of refraction between two media.

For electromagnetic wave (or light), metal with better conductivity will has better reflectivity.