Well, choosing just one band is a bit difficult, but Bathory is always the first name that pops into mind, so I guess I'll go with that. Favourite song is less difficult, since Bathory has an easy magnum opus for me. I love almost all of Bathory's discography, including the trashy little thrash albums of the mid 90's, but Hammerheart, this song in particular, stands as the crowning jewel of Quorthon's work as far as I'm concerned, and a must-listen to anyone who has any interest in pagan or epic metal of any kind.
The build up, the crack of the guitars as they first kick in, Quorthon's rough unpolished viking-esque vocals, the sense of emotion throughout the song, the thunderous "FIRE" that leads to the guitar lead. It's just fucking majesty and still gives me goosebumps after years of listening to it.