Unicron

The biggest Transformer ever. This robot is completely evil and is big. Don't like Unicron? Tough, he'll just eat your planet. That's right, this guy is a planet that thinks nothing of munching on other planets and moons. And then if that wasn't enough, he can just transform into a giant robot and start kicking ass all over. It seems that Unicron can only be defeated with the Autobot Matrix of leadership, as seen in Transformers: The Movie.