Best for quality or best for ...

Best for quality or best for cost benefit?

As you are in the middle east one important aspect to remember is that the client can be a bit difficult in term of material acceptance. Having said that the best material, if cost is really not in question is 316L SS. Now that costs so the best cost benefit is HDPE, for small diameters, cement lined DI for larger ones. However remember that HDPE is difficult to install in a hot climate as its heat expansion is significant and there are not too many qualified contractors that can install these pipes here. Having said all this, for the Middle East, go for cement lined DI. It is a known material, approved by the authorities, and familiar to the contractors. Good luck.