Alicia.
Her singing is great, elegant, sexy and seductive, sometimes breathy, but she never oversings, giving the music a richer feel. She has had some impressive vocal runs throughout the ages, (which cannot be said about Rihanna,) and she can depart from the original melody and glide by on the sheer sound of her voice when she's singing live. She is also one of the very few artists who are capable of pulling off vocal harmonies.
Rihanna’s voice sometimes makes me clutter. She can sound very convincing sometimes, but she can also sound as if she is trying too hard or as if she is bored, and make a song seem faceless.