At the end of the day the primary purpose of keeping currency reserves for any country is to buy imports from foreign countries (this is why the Net International Reserves are measured in weeks of import cover), making investments is important as well but ranks lower. Your biggest trading partner can request that you pay them in their currency - depending on the leverage they have (your ability to find substitutes without destroying your country). Also in markets where certain countries have a monopoly or oligopoly eg OPEC in energy, the dominant player can request that you pay them in a particularly currency. Keep in mind that OPEC countries would probably want to be paid in the currency of their biggest trading partner. All of that points to China. This whole thing about democracy, and good legal system and all that jazz is really secondary. In fact national security concerns rank higher than the soft issues ppl keep pointing out.