In my opinion, the Free Software Foundation, ironically, since they invented the movement, with open source starting out as a tacky rip-off with the ethics stripped out. After decades, open source converged on free software.

More popular opinion is OSI: https://en.wikipedia.org/wiki/Open_Source_Initiative

They were founded by the persons who (claimed to have) invented the term in order to steward it. It's the same definition as the FSF.