Why do you think there is "best" version of Mac OS to run Xcode?
Anyway, it depends on the version of Xcode you use, and you didn't tell that.
More on https://stackoverflow.com/questions/53108403/minimum-versions-of-xcode-and-macos-needed-to-submit-apps-in-november-2018
I currently running macOS Sierra as my operating system on my iMac. Also, I am using the X-code to developing iOS applications as a beginner. I have thousands of questions on developing iOS applications. The first one I need to solve is, I need to know the best Mac OS Version to run the X-Code. I heard some news about Mac OS X El Capitan as the best Mac OS Version to develop iOS applications. All I need to know is the El Capitan is the best Mac OS X version to develop the iOS applications?