The famous Hadeethu Jibreel (The Hadeeth of Jibreel on Teaching the Religion) which contains the best summary of what forms the core of Islam, Eeman and Ihsan by listing out each of their Arkaan (pillars). The narration also teaches us two signs from the Signs of the Day of Judgement.
A very important hadeeth that plays a significant role in formation of a Muslim's life, such that the hadeeth has been even titled has the Ummu Sunnah.