The State of California. However they chose not to challenge and, unless they are ready to open up the court to suits from individuals and groups without standing, they had to rule this way.
So don't blame the Court in this one. Blame the administration in California, and the people who elected them.
I blame the court. With this opinion they gave every governor the power to thwart popular will with a simple executive decision not to defend a law passed, or at least a referendum approved, regardless of how overwhelming its popular support.
There is no certainly in the law. With this decision the states are ruled by men and not by law.