I would say SEASONS

I would say Boys & Girls or maybe evolution

SEASONS definitely.

I would say her signature song is M.

M is the first song that comes to mind when I think of Ayumi although Evolution is a close second.

Perhaps Boys & Girls?
I don't think she has just one signature song.:yes

A Song for XX or Who... for me :)

Can't decide among fly high, SEASONS, evolution or M :laugh

A lot of Ayu's songs from the so-called Golden Era could be considered Her signature ones. I for one always appreciated "SURREAL" the most since it is like the epitome of an unusual pop-song to me. Very strange song structure and so much melancholy in the instrumental part, yet coloured by a very up-beat driven cheerful chorus. "SURREAL" is neither black nor white; it's a mysterious mixture of both.

I will never understand this song in its wholeness, though. Such a weird song, providing the listener with so much pleasure :D

I love your description of SURREAL. Indeed, an unique song. As a signature song, I would choose Moments.