I read somewhere that silver returns more reflected light
than any metal. "
All the large (over 1 m diameter) reflector optical telescopes
use aluminum as the coating on the mirror. The mirrors are
removed from the scope periodically to have the old layer removed
& a new one put on. The aluminized mirrors are open to the
elements when the scope is in use.
Based on this use it looks like aluminum is right up there as
being very reflective. It could also have something to do with
fact that aluminum oxide isn't as dark as silver oxide.