Well... to be a good singer everybody needs to take some vocal training... There are a lot of pop performers that don't really know how to breath, how to use his/her voice, how to sing while moving, how to project his/her voice...
Most of pop singers aren't amazing vocalists and don't have great voices... their voices don't have good range (like... their high and low notes aren't very high and very low compared to their mid-notes) and don't have much potence (they can't sing "loud") but even so, can use it good enough for their own songs... Ayu, Hikki, Namie, Madonna, Kylie, Gwen, etc. aren't stunning singers... But they learned how to take good use of their voices, so, even if they are not amazing vocalists, they are medium or good ones... (like... 3 of 5 stars

).
Rihanna, Avril, Kate Perry, Mika, Koda, Ai Otsuka, Britney... they all aren't very good (0 to 2 stars :B), but even so, they can improve a lot if they take some effort on it (Avril's singing is really better now than it used to be... Koda is improving with time too... Ayumi was terrible back in her 1998-2002 era)
Some amazing vocalists would be... Alan, Ayaka, Ayaka Hirahara, Shimatani, Christina, Beyonce, Celine, Mariah, Kate Hudson... even if I don't like their music or voices (Christina and Mariah, don't like their voices at all), I can't say they are bad singers... Cause they nearly aways are amazing vocalists
By the way... Being a good vocalist is not only about singing in a beautiful way... If the singer keeps singing without any coaching, he/she can damage his/her voice for good... Ayu lost her voice past year because of this at AT08, remember? ^^