Copper, but my it would be expensive and I don't know how it would hold up in your location. The neighbor has a flat copper roof and she was the only one not repairing her roof after a mega hail storm.

Copper IMO will be a waste of money on a flat roof. Whose gonna see it , and hence, the metal need not match the siding. We have a metal roof (not copper). It is seamless and we luv it. I expect never to a have a need to replace it. But, we do hear the raindrops on roof in a heavy down pour. Our roof not very well insulated i guess (we brought house with it already roof installed and inside remodeled. We like that rain sound though so the noise is not an issue with us