Hmmm, her highest selling single is 'A', but because it's a maxi-single, I suppose you'd have to divide those sales by 3.
Her next highest selling single is SEASONS. I suppose you can call SEASONS her biggest hit. It was huuuuuuuge in Japan and a large amount of young people identified with the lyrics.
After that it's M, and then Evolution.
Dearest, Voyage, and No Way To Say were also pretty popular, considering they won the Japan music award. Ayu is the only one to do that for 3 years consecutively I think.