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.
only traditional coin are stablecoin that exist one this days. but in cryptocurrency i think this is not existing because lot of traders do some dump and pumping of coin thats why it will never have this type of coin.
I would say that stable coin in crypto will be any coin with fixing in any currency or even cryptocurrency ( as librecash mentiond above, the price there is fized in ETH) BUT it has to be worldwide used, that requires really professional team and spotless marketing. As for now no project can do that.