I have actually known about the professor with the highest market value for several years. I was introduced when a mentor of mine and I were discussing an "on the way up" colleague and he observed "he really knows how to play the game." As our discussion continued, I realized that most valued means most valued to other faculty and "the game" was largely self-promotion and avoiding controversy. Not that the professor he spoke about was not productive. He was but, to some extent, even his productivity was self promotion. When he submitted an article his first sentence always mention his latest article in the Harvard Law Review. The fact that it was a book review was beside the a point as was his blatent appeal to institutional authority. That appeal was not confined to cover letters -- the class room, causal conversation, and every faculty blurb included use of the H word. As we talked it was also revealed that the professor was a perfect fit for Making Nice, Knowing Better, and Doing Nothing and was a heavy player in the market for tenure letters. The most valuable professor is one who is productive and who "knows how to play the game." Let the bidding wars begin!
That, however, is different from the most valuable law professor from the standpoint of stakeholders -- students, taxpayers and donors. From that perspective the most valuable professor makes a difference in the lives of people other than law professors. Actually, there are many professors who tie for this honor. Mary Reilly has mentioned him or her here already. The actual most valuable in any meaningful sense usually does this: teaches a minimum of 200 students a year and does it carefully, does not insist on teaching 2 days a week to clear time for research or sailing (as a recent commentator suggested), picks topics to write about that have some connection with making others better off (ideally those least well off to start with) as opposed to simply impressing other professors, is more interested in feeling good about the substance of what is written than the status of the review or filling two more lines on a resume, does not make hiring or tenure decisions on the basis of social or political comfort, raises and discusses difficult issues even when the collegiality card is played. I could go on but you have one or more on your own faculty. There are no bidding wars about these professors which tells you all you need to know about the market for law professors.