I love a sexy voice, and my preferences fall into two categories: the baby-doll and the sultry MILF.
In the baby-doll category, think Madonna singing "Santa Baby." That kind of voice always gets me off. Melanie Griffith also does a great one.
In the sultry MILF category, think Brenda Vacarro, Lauren Bacall, Suzanne Pleshette, or Barbara Feldon. Sorry, I can't think of any good examples from this century! It seems to be a dying art.