I used to listen to Yamasaki Ayumi, Nakashima Mika, Mr. Children, and the band who sang "Shima Uta". But I haven't found a new band/singer to listen to.

Upon self-reflection, I find that my favourite songs in the past year are by Mitani Takumi and Yokoyama Daisuke. They sing in a children's program, which my kids love to watch :-p.

I am not a fan of the 48s, but occasionally hear good melodies from them.

I do listen to Jpop, currently in my playlist there are Stereopony, Yui, Aimer

Is there any Japanese rock singer who sings like Freddie Mercury (Queen)?

Yes X Japan and L' Arc-en-Ciel I think.