Copper or Aluminum flashing
Although some may say this is an opinion but copper will outlast aluminum 10:1
Copper over years will have a patina that is either green or brown depending on the conditions. Aluminum if not coated will turn a dull tint and pit over time. Depending where the flashing is used it will also prevent mold growth.
As an example of long lasting I have worked on 100+ year old homes that had copper gutters and flashings that were still in great shape.  I cannot say I have ever seen 50 year old gutters or flashings made of aluminum.