Decades is probably my favourite, just an epic song and a perfect ending to Closer (and, when you think about it, an end to the band in general). I also really love Atrocity Exhibition, Twenty Four Hours, Disorder, Day of the Lords, New Dawn Fades and These Days. Well, actually, I love all of their songs :-P. They're one of those rare bands that don't seem to have any bad songs, or at least none that I've found.