While people are discussing the label polymath in another thread (which reports that the polymathic Noam Chomsky has been cited as, in descending order, a philosopher, cognitive scientist, political activist, and author, but not as a linguist), a letter to the New York Times Magazine (October 18, p. 12, from Andrew Charig of Middlefield, Mass.) laments the death of William Safire, "who most likely was the foremost expert on the American language". Expert?
Ben Zimmer made note, in his "On Language" piece on Safire (on October 11), of Safire's "acute awareness of the limits of his own expertise". Bill never called himself an expert, or a linguist for that matter, and rightly so. He was a journalist who wrote enthusiastically about the English language (getting his information from references, his mail, and the work of his assistants) and expressed opinions about English usage, but he was no kind of scholar and didn't pretend to be one.
(A side point: "the foremost expert on the American language" is unanchored in time. I assume that Charig meant merely that at the time of his death, Safire was the foremost expert on the American language, but he could be construed as saying that Safire was the foremost expert of the late 20th century, or of the whole 20th century, or of all time.)
Now, a question for the readers: who would you nominate as the foremost expert on the American language (in one or another of these time periods)? An obvious candidate is H. L. Mencken, author of The American Language (originally published in 1919, with revisions and weighty supplements over the following decades).
Mencken is an interesting case. Like Safire, he was (proudly) self-taught and strongly opinionated, and journalism was the center of his working life. Mencken, however, developed his enthusiasm for American English beyond the writing of columns, to a systematic survey of the language, thus making himself into a kind of expert, though without an academic association.
Moving to more recent times, we'd want someone who was an expert in all aspects of American English: lexicon, pronunciation (phonetics and phonology), morphosyntax, sociolinguistics, and dialectology, and the historical developments in all of these areas. There are first-class experts in each of these areas, sometimes covering two or three, but covering them all is a hard row to hoe; there is just so much known now in each of them. Bill Labov is probably as close as it gets.
But I'm happy to hear other nominations.