Lunatic Liar or Lord
Jesus made the most astonishing claims, not only about God, society and ethics, but also about himself. He claimed to have the authority to forgive sins, to be the representative of all humanity who came to die in order to reconcile man to God, and to be the only way for people to attain salvation.
"Sin" is not a common word these days. The Bible uses it to describe all the wrong stuff we think and do. Think about it something like this: Imagine a large room, with blank white walls. Then, like an art gallery, images start to appear on the walls, each one illustrating your life. Everything about you. Every single thing you have ever done or said. Even things you didn't do that you should have done. Imagine the pictures appearing one by one, going back over your whole life. In addition, imagine that every thought you ever had, and everything you ever felt, no matter how fleeting, was also captured and illustrated on the walls. Nothing is left out. The gallery represents a truthful, frank and full account of your entire life. There would be some great memories, some wonderful experiences, but what about the bad stuff?

Now imagine that the gallery is opened to the public. Everyone comes and looks at this record of your life - your family, friends, work colleagues; everyone that knows you. How would you feel? Honestly? Would you be comfortable? Would you feel ashamed? Would you be able to look people in the eye? There would be plenty that you wouldn't want people to see. That type of stuff is what the Bible refers to as sin. Mark 7:20-23 "What comes out of a man is what makes him 'unclean.' For from within, out of men's hearts, come evil thoughts, sexual immorality, theft, murder, adultery, greed, malice, deceit, lewdness, envy, slander, arrogance and folly. All these evils come from inside and make a man 'unclean.' "
Have a look at this presentation for an explanation of sin and the answer to the problem.
Coming back to Jesus’s incredible claims about himself, the first thing that we might say about Jesus is that his claims were false and he knew it, in which case he was a liar. Jesus’ claims about himself were so central to his teachings, however, if they were lies then he can hardly be deemed a great teacher. If Jesus set out to systematically deceive people about who he was and how their sins were to be dealt with, then he was among the worst teachers that have ever walked the earth.
Alternatively, the second thing that we might say about Jesus is that his claims were false and he didn't know it, in which case he was a lunatic. If Jesus believed that his claims about himself were true, and they weren't, then he was a delusional egomaniac. If an ordinary person believes himself to be God incarnate, then that person is, put quite simply, insane. Again, if this were the case, if Jesus taught that this is who he was and was mistaken, then he was as bad a teacher as there has ever been.
Finally, the third thing that we might say about Jesus is that his claims were true, in which case he was, and is, Lord. If Jesus believed that his claims about himself were true and they were, then Jesus was not only a great human being, but was also God on Earth.
This type of approach is sometimes referred to as the "Trilemma". There are three options about Jesus: Lunatic, Liar or Lord, to call him just a "good teacher" is not an option. This is covered well at the following Trilemma website.