User blog comment:B214/Question and Answers 6/@comment-25072311-20190401190525/@comment-10330822-20190419022032

Issei has a dream that he marries the girls in groups, which is supposed to be a "vision of the future". So none of them are really "first" stictly speaking, because several get married to him at the same time. If it carries on the EX path, than Akeno will be the first one pregnant of the wives.

Crom and Ddraig ultimately fought to a draw during Rias and Issei's Rating Game with Crom fighting the entire fight with a broken wrist, so its not 100% clear which is stronger. Vali apparently can move faster than Crom can see, so the statement he's 100% stronger than Vali and Issei is sort of up in the air. He may have an advantage, but we haven't actually seen enough evidence to say he's definitively stronger than either. The only thing Crom definitively has over Issei and Vali is fighting experience, which is how he swatted Vali despite not being able to see him, through raw experience.

I'd imagine they are all pretty close right now, because Vali and Issei are now drawing on the DxD forms more than their Heavenly Dragons, so their powers have potential to reach higher than Albion and Ddraig's because of Ophis' influence.