Hardly any of those are singers, they just talk into a microphone and have insane fans love them for their ass. Of the ones that can actually sing:

Beyonce' - Destiny's child
Alicia Keys - the dairy of alicia keys
Mariah Carey - The Emancipation of Mimi
Janet Jackson - Danajo
Kelly Clarkson - BreakAway
Christina Aguilera - Stripped

I'd say Kelly Clarkson, Beyonce, Christina and Mariah are prettiest.