Solo makes it challenging because so many of my favorites are only with a group. (several of these are both with a group and have solo projects).
Mariah Carey
Alecia Beth Moore (Pink)
Eithne Patricia Ní Bhraonáin (Enya)
Sarah McLaughlin
Lita Ford
Floor Jansen
Tarja Turunen
Amanda Somerville
I would so much like to have added Tara!