Rainbow is such an great album, it still is and probably keep being my favourite Ayu album of all times. Though it was also my first - nostalgia!
Got a fan of hers about 2004 with it and I still remember reading about her changing her lyrics style, cause of incorporating English for the first time, and how mad some japanese fans were about it. Also Rainbow did take a different direction in music than Duty and I am... before, more pop orientated and R&B/urban influences here and there as well.
Guess all this took part in the Rainbow dislike back then.
Couldn't understand back then and don't understand now.
The only other album I adore BEFORE Rainbow is LOVEppears.