I think the most important teaching the Prophet (S) has taught human beings is to be a perfect example of being submitted to God and surrender to God's will.
Living in Full Submission to God
Although, he is a good role model and example  for human beings in both social and individual aspects of life, he taught human “how to obey God and how to live in full submission to Him.” This feature, indeed, is the only reason and philosophy behind the creation of human. He reached the highest level that a human being can ever reach. He gained the title of “His Servant” (Abdullah)  as he adjusted his whole life to obeying and worshiping God, the Almighty. ʿAlī ibn Abī Ṭālib (A) is reported to have said, “He was the most generous of people, the most truthful of the people in speech, the gentlest of them in temperament, and the noblest of them in social affability. If someone saw him unexpectedly, he was awestruck by him, and if someone associated with him knowingly, he loved him. . . . I have never seen the like of him, either before him or after him.”
Above features show that how he corresponded with submission to God, the Almighty. He was generous to poor and kind with orphans displaying affection and love to his household. He was merciful to people even his enemies and he suffered a lot while seeing others who suffered injuries, losses and problems. His personality aimed one target: “to be submitted to God in full.”
 Chapter 33, verse 21. لَقَدْ كَانَ لَكُمْ فِي رَسُولِ اللَّهِ أُسْوَةٌ حَسَنَةٌ لِمَنْ كَانَ يرْجُو اللَّهَ وَالْيوْمَ الْآخِرَ وَذَكَرَ اللَّهَ كَثِيرًا
 Chapter 72, verse 19. وَأَنَّهُ لَمَّا قَامَ عَبْدُ اللَّهِ يدْعُوهُ كَادُوا يكُونُونَ عَلَيهِ لِبَدًا
 Nahjul Balaghah, see sermon no: 26, 72, 116, 160.