Iron, duh... Gold is the weakest crafting matieral in the game, also you can do like nothing with it besides crap useless tools, and a clock (Which isn't even that useful).
Iron, you make the 2nd strongest tools in the game, you can make shears, good armor, compass, piston, rail, cart.. It's no competition, iron wins.

Brass.

but seriously, it depends if I'm trying to play spleef or if I'm trying to gather resources.