If we're talking only about japanese artists, then for me Ayu is the number 1 Queen of All the Chart. Ayu is just Ayu.
Second place is for Kana Nishino! I followed her since the beginning, so she's very dear to me! <3
Then there are alan, Namie Amuro, Kumi Koda and Girls Generation (even if they're korean... but they do have a japanese career
), in no particular order. It depends on the time, but they're all ladies I never get tired of and I follow constantly.