Best seraph of the end covers

Which volume has the coolest cover in Seraph of the End?

How this ranking is computed

Every duel won lifts a contender and every loss drops them, by an amount that depends on the opponent's strength: beating the leader is worth more than beating the last. It is an ELO, the same principle as in chess. Duels are not drawn at random — an adaptive entropy sort puts the least predictable match-ups first, which settles the ranking far faster than a full round robin. A worldwide ranking is only published past 30 completed plays. Known limit: a contender who has faced few opponents keeps an approximate score, and the bottom of the table moves more than the podium.