THIS IS REALLY TOUGH but i would definitely say ayu's lyrics for the first four albums were the best... then she started recycling bits and pieces :/

A Song for XX - A Song for XX
Loveppears - too late
Duty - Surreal
I Am... - Endless Sorrow
Rainbow - Free & Easy
Memorial Address - ourselves
My Story - Moments
(miss)understood - Pride
Secret - Jewel
Guilty - untitled~for her~
Next Level - GREEN
Rock'n'Roll Circus - sexy little things