User blog comment:B214/Volume 22 spoilers/@comment-2.102.21.54-20160814202124/@comment-9595671-20160815013339

True, but if we go by EX then I don't think Issei becomes one of the Maou so Ultimate-Class is the last ranking he can get.

Atleast if I'm thinking things right, I have heard God-class but I can't recall if that's only for power or strength based on what Rias said was necessary to beat Riser back in Volume 2.