6. "What If... Peter Quill Attacked Earth's Mightiest Heroes?" (Season 2, episode 2)
Guardians of the Galaxy Vol. 2 isn't looked upon as one of the MCU's greatest entries (although it is significantly better than some of what came after Phase Three ended) but there was real potential in its story - potential that What If...? season 2 decided to explore a bit further.
Its second episode, "What If... Peter Quill Attacked Earth's Mightiest Heroes?", focuses on a timeline in which Yondu followed through on his task and turned Peter over to his father Ego. The little boy then returns to Earth, brainwashed into rebuilding it in his father's image - forcing SHIELD to put The Avengers together a bit earlier than the MCU timeline did.
This Avengers team consists of Mar-Vell, Goliath, King T'Chaka's Black Panther, Thor, The Winter Soldier, and Hank Pym, with Agent Peggy Carter, Howard Stark, and Hope van Dyne also providing some much-needed assistance. And they ultimately make a great team, showing Peter the beauty of Earth again and helping him in the fight against his father.
This is perhaps the most star-studded episode, featuring the voice talents of MCU stars Hayley Atwell, Michael Douglas, Laurence Fisburne, Kurt Russell, Sebastian Stan, John Slattery, and Chris Hemsworth. It's also a great installment in the season, using the "What If...?" concept incredibly well by uniting a different line-up of Avengers while also featuring some familiar faces.