Well, this will come as a big shock, won't it? Over the Hills and Far Away is my favourite here, even though Away and 10th Man Down also are awesome songs. This is probably one of Nightwish's best covers (if not the best, but I'm not sure I can take that honour away from The Phantom of the Opera), and in my humble opinion also better than Gary Moore's original.
Then to the arguments. What makes it stand out more than Away is the fantastic riffs and rhythms used, and what makes it stand out more than 10th Man Down is the positive melody and the fantastic story told in the lyrics. This is one of my favourite Nightwish songs, and one I always feel like rocking away to.