In no particular order, I like the following:
Nickel Silver: For its ease of working/finishing
Stainless (416): for pieces that will be engraved
Iron (and 4140): for pieces that will be engraved and/or blued
Damascus: For higher dollar pieces, and for those situations where it just "fits" the knife.
I have also used Sterling Silver and Pure Silver, as well as Bronze, but those materials are the exceptions.
As a maker, my list is based on materials that not only compliments the particular knife design, but on the working qualities of the material(s). For example, placing damascus fittings on a knife will make it a higher end piece simply because of the additional time and labor it takes to work the material and make the finished piece come out correctly. Take the same blade and handle material, and use Nickel Silver, and the end cost is greatly reduced. Another example is that most makers who intend to engrave the fittings will choose 416SS or Iron/steel for the fittings because nickel silver or any of the softer materials are simply a pain to engrave as the tend to "gall" and tear, rather than cut cleanly. Ask an experienced maker why he/she chooses to use a specific material, and if he/she is being honest, chances are your answer will have something to do with choosing one over the other because of its construction qualities for a given type/style of knife.