Galvatron

Galvatron was once Megatron, but he was then rebuilt from his death throes in a Faustian pact with Unicron. He retains the memories and spark of Megatron, but his personality was significantly altered after the near-death experience, massive physical upgrade, and reprogramming. And all that was before he went insane. The cause and extent of Galvatron's madness varies between different continuities, as does his physical power. Sometimes he's depicted as almost invincible.