While there has always been debate within the DC Comics fandom over which Justice League member is the all-time most powerful, there is only one who stands above the rest to fill that void, and he is the last person fans would expect.
The Justice League is an ever-changing team of DC’s greatest heroes, but some of the main core members include Superman, Wonder Woman, the Flash, Batman, Green Lantern, Aquaman, Cyborg, Hawkman, and Doctor Fate. With that short list alone, it is difficult to name the one who is the most powerful among them, though the general consensus within the fandom is that it usually comes down to Superman or Doctor Fate, as their powers more often go above and beyond what the other Justice League heroes can do. However, neither of them can call themselves the most powerful because of one particular storyline that brings one of the most powerful entities in the DC Universe onto the Justice League.
In the series Justice League Odyssey by Joshua Williamson and Stjepan Sejic, immensely powerful elder gods are threatening life within the mysterious Ghost Sector. A new Justice League team jumps into action; one that includes someone who had, for the longest time, been the team’s all-time worst villain. This new incarnation of the Justice League includes Cyborg, Starfire, Jessica Cruz’s Green Lantern, Azrael, and... Darkseid.
Darkseid is the tyrannical ruler of Apokolips who is considered one of the most powerful characters in DC Comics history because of his New God Physiology. Darkseid is a multiversal entity of pure evil who was fragmented into lesser forms that exist in every conceivable universe, and even those forms are stronger than almost anyone he’s ever faced. He is so powerful that it would take less time to list the powers he doesn’t have, though his strongest and most used abilities are his Omega Beams, which stem from his cosmic Omega Effect power, dimensional travel, and enhanced intellect. Darkseid is so powerful that his death broke reality, turning the League into dark gods until he was quickly resurrected, and he has since been revealed to be the living fist of DC's ultimate villain, the Great Darkness.
Due to his inclusion on the Justice League during Justice League Odyssey, Darkseid is technically the most powerful member to serve on the team. In a world where magic and time travel exist - and in which cosmic beings commonly assemble Justice League teams from across time to solve individual problems, as in Generations Shattered - this status has weight. Should any spell be cast to summon the League's most powerful members, the heroes will likely be dismayed to find Darkseid among their number, especially now Infinite Frontier has returned all past stories to continuity in one form or another.
Not only is Darkseid basically all-powerful, but his victories also speak for themselves when it comes to his power level. Darkseid has been able to defeat multiple Kryptonians at once, making him way stronger than Superman, and his multiversally cosmic nature easily makes him Doctor Fate’s better. Since he’s stronger than the two strongest Justice League members, and since he served as a member, Darkseid is officially the strongest member of the Justice League, as much as that would outrage any given member of the team.