From the Original Transformers Series (1984-1987):
* Optimus Prime: The leader of the Autobots, Optimus Prime is Megatron's arch-nemesis. They have had countless battles, and Optimus has defeated Megatron on numerous occasions, sometimes permanently (though Megatron always seems to find a way to come back).
* The Matrix of Leadership: Optimus Prime is able to channel the power of the Matrix of Leadership, a powerful artifact that grants him immense power and allows him to defeat Megatron.
From the Transformers: Animated Series (2007-2009):
* Optimus Prime: In this version, Optimus Prime is once again Megatron's primary adversary.
* Bumblebee: In a surprising twist, Bumblebee was able to defeat Megatron in a critical battle using his speed and agility.
From the Transformers: Prime Series (2010-2013):
* Optimus Prime: Optimus Prime is once again the primary force behind Megatron's defeat.
* The Autobots: In this series, Megatron was finally defeated by a combined effort from the Autobots, highlighting the importance of teamwork.
From the Transformers: Age of Extinction (2014) and Beyond:
* Optimus Prime: While Optimus Prime has been seemingly defeated by Megatron in some of these iterations, his spirit continues to inspire the Autobots and ultimately leads to Megatron's defeat.
* The Dinobots: The Dinobots, a group of ancient Autobots who can transform into dinosaurs, have also been instrumental in defeating Megatron.
Beyond Optimus Prime:
* Other Autobots: Many Autobots besides Optimus Prime have fought and defeated Megatron, each using their unique strengths and abilities.
* Humans: Humans have played a significant role in defeating Megatron in various iterations. For example, in the Michael Bay films, humans have played an active role in taking down the Decepticons.
* Outside forces: Sometimes, external forces like Unicron or other powerful entities have played a role in Megatron's downfall.
Ultimately, Megatron's defeat is usually the result of a combination of factors, including the power of his adversaries, the strength of their allies, and the consequences of his own actions.
It's important to remember that the Transformers universe is vast and has many different iterations, each with its own unique storylines. The character of Megatron and his defeats have evolved significantly over time.