Jean Grey (Phoenix)

20th Century Fox

Out of all the X-Men members, Jean Grey with the Phoenix Force is possibly the most powerful character there is. We've seen her portrayed briefly in X-Men: Apocalypse and more so in X-Men: The Last Stand, but those versions of her showcase nowhere near her full potential.

In the Phoenix form, she can control time and manipulate matter. In pure form, she is also able to resurrect herself and others. Thanos might tremble in her presence even with all of the infinity stones.