What is the chapter on which the Prophet, sallallaahu 'alayhi wa sallam, said: "A chapter has been revealed to me tonight, and it is better than the whole world and whatever it contains.”

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His Slave and Messenger as well as that of his family and all his companions. 

The chapter you are inquiring about is chapter Al-Fat'h, Quran 48. The Prophet said: "A chapter has been revealed to me tonight, and it is better than the whole world and whatever it contains. Then he recited: {Indeed, We have given you, [O Muhammad ], a clear conquest”}[Quran 48:1]” [Al-Bukhari, Ahmad, and At-Tirmithi].

Allaah Knows best.