I like "A Song for XX" era the least. Mostly because her vocal performance on that album , the remakes of the in my opinion best songs from that album found on "A Best" are a lot better.
for best I have to go with "I am..." , though I find "LOVEppears" almost as good. "I am.." simply has all most of my favorite songs by Ayu
When talking about "Ayu's eras" I myself also like to think about the remix albums , Ayu-Mi-X II - series is what makes the "LOVEppears"-era 2nd best for me.