There have been so many great people throughout history. Since I tend to think of "great" as "revolutionary" in regards to history, I'd have to go with Ahkenaten. He transformed an entire empire, spiritually, socially, artistically, and economically. Although many people think that his change of Egypt from polytheism to monotheism was selfish and cruel, I think that the courage and power it took to do so is something to be admired. There are a lot of theories about his connection with figures from the Bible, as well, but I won't get into that. I'll just say that he was revolutionary and should be admired.
The greatest person that I know is my grandfather. Although there are many great figures alive today and throughout history, my grandfather has affected me more than anyone. He's a patient, kind, and loving person, and the stories he tells about his life always make me smile. He's taught me a lot about human decency, faith in yourself, others, and a higher power, and that hard work pays off.