Here's my list, in no particular order! I love love love those songs, so I hope you like them as much as I do.
Gosh, I wish I could add more songs!
1) Shoko Nakagawa - Kirei.À.La.Mode
2) Shoko Nakagawa - Sorairo Days
3) aiko - KissHug
4) ayaka - Ai wo Utaou
5) ayaka - Real voice (basically, everything by ayaka is excellent!)
6) REMIOROMEN - Konayuki
7) ERIKA - Destination Nowhere
8) Perfume - love the world
9) Hatsune Okumura - Honto wa ne
10) Ami Suzuki - Aozora to Water
As for Ayu, I'd recommend "alterna" the most. "kiss o' kill" too.
And if you're interested in K-Pop, check out:
- Girls' Generation
- Wonder Girls
They are awesome.