SPOILERS Issei Hyoudou’s strongest transformation is easily the Apocalypse AnswerArms (AxA). This form came into being when Ryuuteimaru (Skithblathnir), the familiar of Issei, absorbed the power of Great Red that was sleeping within Issei’s body, how did Issei get this power? I’ll tell you. During the events of Volume 12, Issei was resurrected by both Great Red and Ophis, the two Dragon-Gods, the current flesh of Issei is made out of Great Red’s, and his body is made to work like a human’s through Ophis’s power as well. Throughout the light novel, Issei gets two power-ups through their power, these would be Dragon Deification and Apocalypse AnswerArms, Dragon Deification (DxD G for abbreviation) uses the power of Infinity that Issei gained from Ophis and uses that within his True Queen armor, though Issei’s body couldn’t take that power and had to make a weaker Pseudo form, which would be named Pseudo-Dragon Deification (P DxD G). Later on in the events of Shin High School DxD Volume 3, Issei’s familiar started transforming while using the power of Great Red, later on being turned into Apocalypse AnswerArms, which has the aura to destroy a planet, this being Issei’s strongest transformation. Issei had also managed to unlock True Dragon Deification (T DxD G) in the events of Shin High School DxD Volume 4 which is the latest volume as I’m typing this, which is the strongest armor form of Issei so far. Issei will more than likely become even stronger in quite a few ways. Issei has yet to unlock the power of the 12 Heavenly Breasts told to him from Chimune Chipaoti. It is entirely unknown what kind of power Issei will gain from this. Issei also will get power to control the AxA. The way he will be able to control that power is from the power of Eros, the Greek God of love, the power gained from this will be called Eros Engine or ExE. We have also yet to know what the effect of Eros Engine and True Dragon Deification will have on AxA. Also Lucy and Beel wouldn't Stand much of a chance if we're talkin end of the series there's multiple versions of Lucifer's in the Multiverse and she's the probably the weakest hell Lucifer from Supernatural and Lucifer could take her out.You can have your own opinions but Issei's the harem god that will never change.