Zemo managed to cause a bombing massacre through the Winter Soldier and successfully tore up Avengers until Endgame. Even when he was imprisoned, he still noted that his plan succeeded for the most part.

Possible SPOILERS:

Zemo. Even though his actions were the reason why the Avengers failed to defeat Thanos in Infinity War (mainly) since it divided the team and they had to be united in order to win.

I said Zemo for obvious reasons, however, his plan was for the Avengers to die from within. That happened but it ended when they reunited in Endgame. On the other hand, Quentin Beck reveals who Spider-Man really is... and that can’t be undone, so who actually won?

Good point actually. However, Zemo’s actions are what screwed the universe. While Beck was posthumously success with his goal, his actions didn’t cause half the universe to die.