Most liberal state on social issues?

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Depends on what you mean by "socially liberal." "Social issues" is just a catch-all term for a bunch of unrelated issues that aren't directly related to economics. Overall though I'd say Massachusetts, despite a slight socially conservative undercurrent. It's definitely not California. If guns weren't a factor I might say Vermont.

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California.  The prize WOULD have gone to Massachusetts, given they were the first state to legalize marriage equality.  However, their transgender protections are still lacking in a couple areas.  California, however, has all of the social liberal and SJW bases covered: lax abortion laws, marriage equality, comprehensive discrimination protections for everyone in the LGBT spectrum, gun control, legalized marijuana, and "affirmative consent" laws.

How are Massachusetts's trans protections lacking?

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This a hard question to answer, becasue "social issues" and "socially liberal" could mean so many things. For example, Intell here has Vermont as the most liberal state, but Vermont is also one of the most gun friendly states, so if you consider supporting gun control a major part of "social liberalism" your list would look different.

A bit of an exaggeration; Vermont is pro-gun for a state as Democratic as it is, but it's not like Wyoming or anything.