initial response would be black, but I recently got a love for gold.
I have a gold/black H belt and i thin the gold side looks awesome against blue denim, especially when wearing a while top cos it matches the top stitching.
It would depend on your coloring. Black haired, blue skin toned people look best in black. Fair blondes look great with gold. Brown haired, olive-skinned folks look great in brown. Dark blondes, I like in grey.
I have both black and gold, and I would say its a toss up. Both colors go with virtually anything. Gold is more of an all year round color - black is the ultimate neutral but it can look a little heavy in the dead of the summer.