1. "What If... the Emergence Destroyed the Earth?" (season 3, episode 5)
Without question the best standalone episode of What If...? season 3, "What If... the Emergence Destroyed the Earth?" did exactly what it said on the tin, imagining a world in which the rise of Tiamut quite literally shattered the Earth into pieces. And in that dark time, Mysterio looked to capitalize on what was left of humanity, beating down the remnants of the resistance that rose up to fight against him and using the White Vision to carry out his bidding. That is, until Riri Williams stepped up to stop him.
While Dominique Thorne was a wonderful addition to Black Panther: Wakanda Forever as the character, it's been some time since she's been on-screen, so this was an epic way to reintroduce her. Riri was seen as the world's last hope, the only person capable of outsmarting Mysterio and ending the war once and for all. And she proved herself entirely, fighting alongside the Resistance and figuring out a way to depower the White Vision in just minutes (before then bonding with the technology that made him, becoming a new version of the White Version herself).
The thing that pushed this one over the finish line was how it played into the overarching arc. Any interference from The Watcher would have been a powerful statement, but doing so here felt incredibly earned. He couldn't watch Riri fall at the final hurdle any longer; he needed her to win so that there was something left fighting for. There had to be more than eternal loss. And with The Watcher's guiding words, Riri made it so.
An emotional tale that evoked all the feelings it wanted to, "What If... the Emergence Destroyed the Earth?" is perhaps the best example of the sheer brilliance that What If...? is capable of when it really commits to the stories it's telling and the ambitious concepts it has created. It also managed to make the most powerful episode of the season the first domino to fall on the way to the series finale.
Now that's some darn good storytelling.