The most logical answer would be obi wan, he is what it means to be a jedi, the epitome of patience, understanding and wisdom. It was through his guidance along with yoda the galaxy was saved from the empire.
That was my first answer but Ill put a second answer that many of you will not agree with but here it goes...Kreia.
Kreia didnt want to destroy evil, you can never destroy evil because it always comes back; the force always needs to balance itself. Kreia wanted to destroy the root of all evil, the force, and in doing so instead of a force controlling everyone people would instead finnaly be free. This would of been like an ultimate victory that the jedi never seem to be able to achieve and thats why kreia was in my opinion the greatest jedi.