Agree on Bjork, I love how she pronounces the 'r', like when she sings 'hearrrrrrrrt' in Unravel... or when she says 'grrrrrrrazzzzzzzzias'
I think musically the most beautiful is Portuguese, especially the Rio dialect/prinunciation... Just remembered this clip, the Blower's Daughter cover in Portuguese:
http://www.youtube.com/watch?v=CjmLI0VyLmM