For I know, the most stable coins is not exist. Even though the USDT and QC( USDT is equal to US dollar, QC equal to China RMB). This is a investment market, so the absolutely stable is hard to maintain.
Stable coin is a term used in cryptocurrencies meant to hold stable values. For example, Tehter (USDT) is a block chain based asset meant to trade for $1 (USD). It is a "Price-stable cryptocurrency."