Having looked at many ...

Having looked at many systems across the world, I have seen many successes and even a few disasters. Obviously there are aspects which relate to the size of the system and the larger the pipes get then the more complex the solution. I have seen steel, cast iron, ductile iron, galvanized iron/steel, uPVC, concrete, fiberglass, MDPE and HDPE (and even brick gravity aqueducts).

For the larger pipelines ductile iron is preferred as it has the durability of cast iron with some of the flexibility of steel. Never use steel unless it is very well protected both inside and out.

The UK went to MDPE for distribution systems many years ago but the world-wide preference has been for HDPE. uPVC can present problems if there are sudden pressure changes whereas PE is more able to withstand such conditions and is easier to use on-site. In Russia, I found a supplier making fiberglass distribution pipes which required the system be taken down for 24 hours whilst a simple connection was made. PE can be connected to under pressure with no shutdown.