I want to replace my mailbox and front porch light and want to know
which metals weather best.

How do these different metals withstand the elements: stainless steel,
satin nickel, aluminum, iron and brass?

Thank you!

Those metals will all last longer than you and I will.  Iron rusts, of
course, but there are liquids finishes that will bind the initial rust
so that the iron does not continue to oxidize (if you don't want to
use paint).  Brass and aluminum will tarnish, losing their original
shiny surface.  Stainless steel and nickel will be least affected by
the elements.