Which pipe material is the best for water supply networks in terms of quality of water supplied ?

Dear Colleagues, I am Davis working on project and construction management, planning, monitoring, control and execution of various infrastructure projects in the Middle East. For my new water supply project I would appreciate to some inputs from you. As we all know, different materials are used for water supply networks such as DI( ductile iron), HDPE, UPVC, GRP, GI (galvanized iron), Copper  etc . I would like to know your opinion about which material  is the best for maintaining the quality of water supplied to the public , irrespective of the cost. What are the reasons behind it? I I'm interested in knowing some recent trends in using these materials, some case study will also be nice to discuss if some one has it. Many thanks, Davis Augusthy Project Manager Muscat, Oman