If you go all-time, my pick would be Albert Davis from Alcoa in the mid 60's. I've been watching high school football practically every Friday during the season since 1959 and he's the best I've ever seen.
More recently, but probably still too far back for this discussion would be James 'Little Man' Stewart at Morristown West around 1990. He was a great player and was UT's leading career rusher when he graduated even though he played with two other pretty good backs there in Charlie Garner and Hayden.