Majin Buu. The child like creature who nearly succeeded in eliminating humanity.

"Now Buu maaad!"

 Majin Buu is the final antagonist in the Dragon Ball Z anime series. With this in mind, fans of the series can reason that he is the most powerful of all the Z Villains. When Buu is freed, he nearly destroys all of humanity and even manages to take down quite a few Z Fighters before finally being defeated by Goku. His regenerative abilities and power of absorption combine with his ignorance to create a truly terrifying foe.