This is very difficult. I love most of them and not just "Dearest."
My will -- Very nostalgic for me.
Fukai Mori -- Sweet, but not a favorite.
Dearest -- Need we say more?

Every Heart -- Love its sweet sound (and it's nostalgic), but its repetitive melody is quite boring after a while.
Shinjitsu no Uta -- Beautiful, but sometimes the voice at the end of the chorus(es) irks me.
Brand-New World -- One of my absolute favorites. The lyrics, the meldoy, the sound... everything!
You know what? Depsite the fact that "Dearest" is here and that this is an Ayu forum, I have to vote for "My will."