Sarah Mclachlan (ironically probably more known for the vocals on "Silence" by Delerium than her own works like "Sweet Surrender", which is one of the most beautiful vocal performances I've ever heard. At one point around 10 years ago I listened to on repeat for a period of weeks usually several hours a day, and I'm not joking.)
Jewel
Stevie Nicks
Norah Jones (not really a fan but was very impressed at Glastonbury... perfectly suited to chillout, which is different from most of the others I like)
Kelly Jones
Mariah Carey (again not a big fan, but I'm in awe of her on tracks like Fantasy)
Top three are probably my faves