User blog comment:B214/Volume 22 spoilers/@comment-66.50.41.60-20160926041302/@comment-9595671-20160926073121

Ddraig says he considers Belzard his strongest wielder because he beat two Hakuryuukou's in his time, and unfortunately he and all of the Boosted Gear's past possessors lost their souls to protect Issei's.

So the only one who could easily answer that question for us is no longer alive anymore. He's definitely not the weakest anymore, but he's not the strongest yet either. Though one could say as of EX's time he's become the strongest Sekiryuutei.