Even though it's not metal, one of my favorite covers is Frank Zappa's live cover of "Stairway To Heaven." It's much better than the original. There's two professional singers, one for the lower part, one for the high part. They speed up the song after the solo, which adds so much to the song. It's Frank Zappa, so....And they do the solo with a group of woodwinds/brass players. Need I say more?
And I'm not thinking well because it's late at night, so I can't add anything more to the discussion right now.