Which metal resonates for the longest when struck?

I don't think it depends on the type of metal, but rather on the structure. An iron board will not resonate at all, nor will an iron rod if it is unsupported. But if you support an iron rod at both ends, it will resonate according to how well it is supported (the tighter the support, the longer the ring tone). Generally in orchestras, steel or iron bells are used in percussion sections. I forget exactly what material vibraphone bars are made of, but they have a ring tone of about five seconds. The wire strings of a piano have a ring tone of about 15 seconds (when struck loudly), and the best wind instruments will produce a good pitch for as long as you can blow into them.

Of course it depends on the metal as that is what the question is asking for!
But what you say does raise the question of whether different metals will be the winner for different shapes, e.g. one metal may form the most resonant wire but another may form the most resonant cymbal or bell.
The answer must relate to elasticity so, for example, I don't imagine gold being the best resonator whatever the shape. Any advance on the 15 seconds of an iron piano string.

I think gongs are made up of copper and brass, or possibly nickel, but I'm not entirely sure. In any case, their ring tone can be 30-60 seconds in ideal conditions.

And if it relates mostly to elasticity, how come a lot of these instruments are not made with more aluminum?

What does resonates mean?

It means a reverberation in the sound. If you struck a piano wire it would resonate - continue to vibrate - for up to fifteen seconds (depending on how strongly you struck it of course).