User blog comment:B214/Volume 22 spoilers/@comment-66.50.41.60-20160926041302/@comment-1359322-20160926074226

I can think of 2 reasons why Issei isn't considered the strongest Sekiryuutei.


 * 1) Issei isn't considered the strongest Sekiryuutei is because he powers-up through unconventional method instead of just relying on Ddraig's power like the past Sekiryuutei.
 * 2) Issei can't use his powers properly yet. He is still far from mastering the Triana, CCQ and DxD G. Not to say that he's weaker but perhaps compared to Belzard who may have mastered Ddraig's power to the max, Issei still pales in comparison.