Well the best runic hammer in the game would be a valorite one, and it will cost a LOT of gold. I think the best bang for the buck is either a gold or agapite hammer, you'll still make some nice stuff with them, but the chance to get something uber will be lower than a val or verite hammer. Unless you want damage spread across certain resistances (say you don't want to cast consecrate), you can do fine with standard iron ingots. If you want a specific trait, look on stratics to find which metal will do that and craft it out of that metal.