I'm not including any J-pop on my list, and I'm not going to list all the big stars like Madonna , Beoynce , Britney & Mariah. I liked all their albums a lot, but people already know them well enough.
I'm mentioining two european releases I've liked the best over 2008:

Rockferry by Duffy
very retro-style album full of great songs. "Mercy" might be a somekind of hit in the US too, I think it's on several big movie soundtracks. but IMO "Mercy" is the worst song off the album.
recommened songs:
- Stepping Stone
- Warwick Avenue
- Delayed Devotion
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Melody by Sharleen Spiteri
Sharleens first solo album, she used be the vocalist of the very succesful band Texas.
This album is also very retro-pop/rock in style, but it's more of a mixed bag than Rockferry. There are songs inspired by old Motown style to 60's brit-pop.
recommended songs:
- I Wonder
- Day Tripping
- Where Did I Go Wrong