Totally agreeing... The Leper Affinity is da' evil song, and is such one of the greatest Opeth song, generally. If I had to choose ONE song to represent them, it'd be this one.
EDIT : That's so funny... I just began re-reading some Opeth song and just realised it talked about murder and things like that... I never thought of it, all I had in mind is that they were very beautiful songs.