Not a simple question. A good philosopher must do his best to honestly answer and ask questions. Avoid the rhetoric and aim for the truth. No easy task. Personal feelings are to be put aside and efforts towards the truth, whatever that means, must be made.
That being said a GREAT philosopher, at least to be acknowledged as great, must create/discover/realize some truth or possible truth/idea and spread the study of that idea. Plato did this by opening his academy. Aristotle did so inadvertently by leaving us his lecture notes. Many more modern philosophers left great texts for others to stud and follow their line of thinking.
What made these men great was the power of their ideas and of their minds that created them. I would say that even if they did not leave behind these idea they would be great philosophers, merely attaining such ideas and understanding shows them to be great, but we can recognize this because of their writings.
It takes a great mind to make a great philosopher. A life of effort and pursuit of Truth are the paths one must take to reach this greatness. No matter if the Truth is found or not, any progress that can better the understanding of what truth is or where it comes from will make a philosopher great.