The US dollar is the standard currency for international trade, but the US government of course has great influence over its use. For example, they sanction anyone who does business with Iran.

This tactic, used against Russia, Iran, and others, has turned them to seeking other, safer currencies. The Euro is risky; EU members are American allies, generally speaking, and also may act against Iran, etc. for their own reasons. The most widely used currency and most stable economy (an unstable economy causes and unstable currency) is the Chinese yuan or renminbi.