This is a tough one not becuase of content but due to personal "listening enjoyment". In a strictly musical sense, Deliverance is my least favorite. BUT the poor quality recordings of the first two albums, and my dislike of Mikes mid-range screams make those a far less "enjoyable" listen. Those first two are frustrating. So many neat ideas and riffs, but so little cohesion and, IMHO, sub-par vocals (technically decent, but sound like shit) really kill it for me.
Im still going to pick Deliverance, and although i think its a good, solid album with great individual segments, i think it falls particularly short of the three classics preceding it.