Such a fun thread idea, just going to bump it, sorry not sorry
The most obvious answer is A Song for ×× to LOVEppears; A Song for ×× is a cute enough album with some cool songs, but LOVEppears was a shocker that catapulted Ayu straight to queen status. An argument for another jump between LOVEppears and Duty can also be made, imo - Duty is where we started seeing the diverse range of styles that Ayu would fill her albums with over the next few years, which I always felt was her strength and what sets her apart from so many other J-pop artists (of course, she is also just
really, really good at it.
Colours to M(A)DE IN JAPAN (is that fair?) is another for me. Colours is middle-of-the-road and generic - listenable but not very exciting. M(A)DE IN JAPAN takes me back to classic Ayu times, such a fun, diverse album with great compositions and some fun productional choices.