My absolute complete favorites are..
Home Made Kazoku - They have SUCH addictive songs! Their use of the English language sounds kind of real too, not just random hyper words stuck in for the heck of it (which I think is cute, don't get me wrong).
Utada Hikaru - Her music (for Kingdom Hearts, especially, but it applies to everything else too) is SO lovely, it's like a dream *-* She's amazing, and her voice is so soothing.
YUI - OH. MY. GOSH. YUI is so adorable and sweet and modest and her music is gorgeous. I'm addicted to at least half her album, my all time favorites being Rolling Star and I Remember You ♥♥♥
Maaya Sakamoto - Her voice is so clear and light! Her RahXephon and Wolfs Rain works are so PRETTY. ♥
I'm sort of torn between the other artists, but I adore the people mentioned above o3o