Fergie for me.
After liking "London Bridge" for "forever," I did not even realize it was her singing on the song (and she even announces her name at beginning of song, ha!) and "I Gotta Feeling."
She doesn't even sound like the same singer to me in these three songs. I love that.
Unfortunately, even though I immensely enjoy an extremely wide variety of music and an equally extremely wide variety of singers, I do not possess the nuanced ear to distinguish subtleties that you mention in your question.
:(