Dr John Stockton wrote:

What's the biggest-scoring legal move in Scrabble?

I suspect that the answer is not known for certain. Here are some
excerpts from Gyles Brandreth, _The Complete Book of Scrabble_, Robert
Hale, London, 1980:

"The highest known score achieved at a single move in a real game is 374
points. This was achieved by playing the word QUETZALS across two
triple-word-score squares, and getting Z on a double-letter-score
square. This score was made by Darryl Francis, of Wandsworth, London,
on 12th September 1973.

....

In a idealized game, the highest score yet discovered for _one single
move_ is 1961. This idealised move was devised by Ron Jerome, of
Bracknell, Berkshire, in May 1974. The move involved playing all seven
tiles, combing them with eight already on the board, to form a 15-letter
word, which stretched across three triple-word-score squares. As well
as making this 15-letter word, the move also created seven words
perpendicular to it, three of them stretching across triple-word-score
squares.

The 15-letter word that was played was BENZOXYCAMPHORS. The seven other
words were DAFFODILLY, GULLISH, JINNYRICKSHAWS, PROVERB, SQUANDERMANIAC,
VAGABONDAGER and WERTUZ. (The words DAFFODILL, GULLIS, JINNYRICKSHAW,
PROVER, SQUANDERMANIA, VAGABONDAGE, and WERTU were already on the
board.)

BENZOXYCAMPHORS - Webster's New International Dictionary, Second edition
DAFFODILL - Oxford English Dictionary
DAFFODILLY - Chambers
GULLIS - Oxford English Dictionary
GULLISH - Chambers
JINNYRICKSHAW(S) - Oxford English Dictionary
PROVER - Chambers
PROVERB - Chambers
SQUANDERMANIA - Chambers
SQUANDERMANIAC - Webster's New International Dictionary, Second Edition
VAGABONDAGE - Chambers
VAGABONDAGER - Oxford English Dictionary
WERTU - Oxford English Dictionary
WERTUZ - Oxford English Dictionary"

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