Invented logic, clarified areas of philosophical inquiry and in particular made the key distinction between natural philosophy (including physics and metaphysics) and ethics, something that Socrates and Plato confused completely. Systematized the areas of knowledge - philosophy and sciences - that have stood the test of time, far more important than Plato (a more or less contemporaneous "competitor").
He benefited from the discoveries of all his predecessors and developed a new coherent philosophical system, which has been inspiring for all our sciences ever since. There was no discipline of philosophy which he wasn't interested in, and to all he gave the foundation, if not a great development also.
No, no single philosopher is or can be or ever will be the greatest philosopher. Each individual philosopher contributes to the vast 'library' of knowledge in our world and each of their ideas are unique. Secondly, there can be no greatest philosophers,only great ones. Lastly, morality does not limit a 'great philosopher', a great philosopher would be able to think about both sides of an argument non-biased.
I think Aristotle would have said that there is no "Greatest Philosopher", only many good ones. Think about it. There are hundreds of philosophers in the world, many all making very valid points in their studies. So no one can say that Aristotle is the greatest philosopher anymore than I can say I am the greatest philosopher.
The issue here firstly is personal bias I disagree with aristotle on many things. Secondly define greatest, most positive impact, smartest? Personally I prefer Socrates or maybe Machiavelli, but I would not call them the greatest philosophers. He was perhaps a great scientist and teacher but was he the greatest philosopher? No.