Lee Hyori (feat. Park Ji Yong) - Amor Mio
When this song first came out, I listened to it over and over again until I practically knew the lines and harmonies inside out. It's such a beautiful, emotional ballad and while Hyori sounds stunning her duet partner totally steals the show for me. I hadn't heard of him prior to this, but I was completely floored when I first heard his gorgeous voice.