An efficient free market maximizes production. Markets in which companies can externalize costs or engage in monopoly behavior produce less than markets in which these behaviors are prevented.
"Free market" is not synonymous with "anything goes." It's a specific economic (rather than ideological) concept. Most economists believe that certain rules and restrictions are necessary to have any functioning market at all.
The usual economic definition of efficiency would be maximizing utility experienced by market participants, not maximizing production.
Currency-denominated "product" measurements are a common proxy measure for utility, but that's a compromise to the inability to measure utility directly.
Wouldn't that be the most efficient free market? One in which only the most brutal and powerful ideas and companies prevail (with money)?