During the Dark Phoenix Saga, Jean Grey's power surged to cosmic levels, capable of annihilating entire planets. While the Justice League has faced threats like the Anti-Monitor, Jean Grey's ability to tap into the Phoenix Force could easily overwhelm even the most powerful members. Her telekinetic and telepathic abilities, combined with her near-limitless energy output, make her a one-woman army. In a battle, she could mentally incapacitate Batman or Wonder Woman while simultaneously unleashing planetary-scale destruction. The Phoenix's ability to absorb and redirect energy means she could neutralize Superman's heat vision or Green Lantern's constructs with ease. Jean Grey's greatest threat isn't just her power—it's her unpredictability, a factor that could leave the Justice League scrambling to adapt.