Shiki "rivalled" Roger and Whitebeard. But people forget that Roger only had one ship, as did Whitebeard at the time, while Shiki had a small army, and an Armada of 50 ships. The might of the Roger pirates was so great that Shiki losing just half of his fleet was enough for their battle to be considered a draw. Imagine Shiki trying to fight against the Roger Pirates without his army behind him. Shiki is definitely a Top Tier, but in terms of individuality, Roger was stronger.