One of my favorite songs is A Forest by Carpathian Forest. The only problem with using this track from Black Shinning Leather/ Through Chasms and Caves, is that it really doesn't represent the music of Carpathian Forest. It's a great dark, mellow song that is more electonica than metal. I think this proves that these guys have a talent for something other than what they normally play.
Almost any song by Therion. I'm kind of partial to the song To Mega Therion from the album Theli. It's like an Opera, clean vocals and some fat lady singing type vocals.
Although Beyond The North Waves from Immortal's Sons Of Northern Darkness album isn't "nicer", it is a very well written piece. Very powerful, and melodic.
Have you ever listened to Nokturnal Mortum? Black Moon Overture, the first track from Goat Horns is a great piece. It is classical music to me. No singing at all, just an orchestral piece.
I don't know if I was headed in the right direction, or not.