In terms of raw power it's obviously Zodiark. He was made to be the Ancients' collective concept of God. And every time he's been beaten, it's due to having his power reduced / being attacked with attacks conceptually created to beat god. Kind of like the Rune of Death and the Elden Beast in Elden Ring. Something something for the World of Man to mean anything Man must steer it something something Gaius, Thordan, and Venat beliefs etc.

In terms of actual power, it's still Alexander. Turns out, time travel within the setting is instrumental in us winning. Both in SHB and in EW. It's a power that apparently the Ascians did not possess or think to try to attempt. And apparently no other civilizations out in space either. If Alexander has the knowledge and will to, he can go back in time and kill any of the most powerful things as fledglings. Something something, cop out answer. Something something, blame the writers not me.