This is at the same time a really hard choice, and a very easy one. Ever since I first heard it I've said that Ghost Love Score is my favourite Nightwish song, and I still stand by those words. That song was one of the first Nightwish songs to get my heart racing, and also the reason I lost my voice the day after Nightwish's last concert in Hartwall Areena 19.9.2009: when I heard the intro I screamed until I was out of air. There was no way of stopping myself, and I wasn't even aware that I was screaming until after several seconds.
So, yes. Ghost Love Score is my favourite from Once, and also my favourite Nightwish song overall. But let it be said that Once also is my favourite album, and that I simply adore the rest of the songs on the album. Especially Dark Chest of Wonders, Planet Hell, Creek Mary's Blood, The Siren, and Kuolema Tekee Taiteilijan have a special place in my heart!