He is one of only a handful of Jedi that can become one with the Force upon his death, a skill that he learned from his dead master, Qui Gon Jinn
He's simply the best!
Obi Wan defeated some of the most powerful sith including Darth maul, Anakin, and Anakin as Vader again through Luke and the force connection established after his death. He also had diplomatic abilities beyond that of most of the jedi counsel.
Yoda once said "Best of all, Dooku would be". He would have been one of the most powerful Jedi had he not fallen to the Dark side.