I first started listening to Ayu back in 2000 when Napster was free/illegal (lol) but it wasn't until the evolution release that she was actually established as my favourite artist. I really love her earlier stuff from around that time and before, it was her voice that captivated me because it was so... unrefined. It was "unique" as panda87 said, and that really stuck with me. She's been my favourite artist ever since. I listened only to Ayu for a good 2 years if not more before i even began to listen to other singers.
I may have said in the past that she's no longer my favourite, but I know that's not the truth. No singer is able to pull be back so often as Ayu. I'm even sat here ripping her albums to my computer right now