Demon of the Fall was the first "heavy" Opeth song which I loved, it's the one song that gets a lot people into Opeth (me for intance):). Now I think that April Ethereal and When are better songs but I would love to hear "Demon of the Fall" live.
I often sing along Opeth songs, When, Godhead's Lament and A Fair Judgement most often. Especailly "Searching my way..." part is so awesome to sing along.
I really like Nightwish and I am trying to find some similar bands. Tarja is an amazing singer and their songs are really emotional, IMHO. I think Oceanborn is their best album.