The Diamond Messiahs are the strongest versions of the mainline protags, so it can't be them, and SJ's protagonist never fought anyone on par with YHVH.
Devil Survivor 1's best feat was beating Lesser YHVH Metatron and Lucifer (depending on New Game+ canonicity).
They mentioned Devil Survivor 2 was a mess to scale to mainline.
Raidou beats Lucifer as his best feat.
Imagine's best feat is beating Satan.
Not sure if IF, or 200X, or the other Summoner series have anything special.
Okay, so… a lot of this is loud and wrong.
The Diamond Realm Messiahs, with the exception of Nanashi, are all different people from differently timelines, who were killed by YHVH.
King of Bel was stated to have fought YHVH, and current Abel is rumored to surpass him.
Devil Survivor isn’t 2 mainline.
Raidou subdued Fury Masakado, take note on the word
subdue, as he had no intention of fighting him seriously.
Haven’t seen Imagine.
Tadano can wield a sword only capable of being accessible by being comparable to YHVH.
Tamaki Uchida defeated Hazama, who manipulated and controlled the entire Tree of Life.
Devil Summoner Kyouji defeated Sid Davis and the Demiurge.
SMT NINE defeated their interpretation of Demiurge, who scales above Base Lucifer.
Stephen, bar the Great Reason, is the strongest entity in Megami Tensei on the SMT side.
it could be possible that Persona MCs are the strongest humans in MegaTen
Considering if you put Kazuya, Aleph, Demi-Fiend, Tadano, Flynn, and Nanashi in a fight against Naoya, Tatsuya & Maya, Makoto & Kotone, Yu, and Ren, the side that would still be alive are the Persona-users.