I love Ayumi Hamasaki, she's my favortie. But others that I could recommend...there is this band I love called Fictionjunction Yuuka. They're a classical-ish band but it does have singing
Another singer I really am fascinated by is Olivia Lufkin. She's well known for doing music in the Anime NANA. She's the one who sings for Reira. I recommend her because of her wide voice range. Her high notes feel almost dreamy kind of. Some songs I would recommend of hers would be A Little Pain, Starless Night, Winter Sleep(actually in English now that I think of it...), and Tears & Rainbow.
Now if someone listens to some of this music, they'll know what I'm interested in which means they can recommend something to ME.
I listen to Ayumi Hamasaki all the time and I need some variety lol >//<