'The Drapery Falls' the defining moment? Definitely not for me, although this song kind of defines when their best period, their artistic height, was over.
For me, the track that truly defines Opeth is 'The Moor', because it has everything that is Opeth within it's twelve minutes playing time: acoustics, beautiful clean vocals/clean vocal melodies, interesting and complex drumming and riffing, growling, howling, a great, dark lyric...the only thing it perhaps lack is that immense atmosphere which Opeth managed to create on 'My Arms Your Hearse'.