Even if Socrates is not the greatest philosopher, he is certainly a foundational figure in the tradition of philosophy. Had there been no Socrates, there would have been no Plato, and then no Aristotle. As Plato and Aristotle were immensely important to the development of western thought, Socrates deserves a great deal of credit.
The oracle at Delphi claimed that Socrates was the wisest man alive. Socrates himself took this to be true because he alone knew that he was ignorant with regard to the most important things. His avowed ignorance led him to question so many of the people reputed to be wise in Athens.
I do not think his method of questioning made Socrates great. Rather, I think it was his humility with respect to the topics he asked about. If we take Plato's characterization to be semi-accurate, he seems to have been open to any answer, including answers that proved his preconceived notions wrong or made him look ignorant. He was willing to investigate, upturning tradition in the process if need be, because the he believed that what was true should guide inquiry (This is especially the case in Plato's Euthyphro). Without the view that truth should override tradition and preconceived opinions, most of the science and humanistic thought that follows from Socrates would not have taken place.