I can't possibly just pick only one.
I really love Ayu's ballads but I can't write them all down, so this these are just a few that I like.
Ayumi Hamasaki
Moments
Dearest
HAPPY ENDING
Memorial address (This is considered a ballad right? xD)
Koda Kumi
you
Yume no Uta
Unmei
Cinderella before midnight (even though this is a cover)
Namie Amuro
Baby Don't Cry
The Meaning of Us
Four Seasons