Your choice of airline type and airline fittings will depend on the type of compressed air installation you are working with, and it’s sensible to know from the outset which materials you plan to use.
The airline fittings will naturally correspond to the type of airline itself, and for instance you might find simple clips to be more suited to flexible hoses, with more robust couplings for galvanised pipework to be fixed into place.
In general there are several types of airline that are recommended for use, and the galvanised pipes mentioned above are a good mo option too, although changing the layout of your pipe later can be more difficult due to the need to drop the pressure of the entire system in order to do so.
Stainless steel is a good alternative to black pipe, bringing many of the same positive attributes, along with better resistance to moisture in the compressed air supply it carries.
And copper pipe is particularly good for clean air supplies that will remain free of particulates over long periods of use.
Ultimately it will always depend on your specific use for the system – but being aware of the different airline options, and the corresponding airline fittings to secure them into place, is the first step towards knowing what you need.