Aluminum, also known as "dibond" or "aluart", is the material that you choose if you want the best of the best. If you've ever been to a photo museum, you've seen aluminum up close. Works are printed directly on the aluminum which are white oven-lacquered with polyester. Dibond is dimensionally stable, UV and moisture resistant and suitable for the long term.
Some of the works also come in a brushed aluminum look; this provides a metallic effect in the white batches of photos.