You did not specify what book you are referring to in your question. The answer, of course, is a matter of opinion. Many people would agree that the most important verse in the Bible is contained in Matthew 7:12. This is how it is written in The King James Version:
Therefore all things whatsoever ye would that men should do to you, do ye even so to them: for this is the law and the prophets.
A different version of the same idea is found in Luke 6:31.
And just as you want men to do to you, you also do to them likewise.
A more common statement of the same idea is:
"Do unto others as you would have them do unto you."
Evidently this advice is so sound and has been so well received that it is known all over the world in practically every language.
The above answer is very good: the Golden Rule is a very important part of what Jesus talks about in the Bible. However, another opinion would be that John 3:16 is the most important verse. It states, "For God so loved the world, that he gave his only son." This is important because it is central to the Christian belief that Jesus is the son of god. Interestingly, Muslim beliefs include Jesus, but they see him as a prophet who was divinely inspired just like Moses or Noah. Judaism also sees Jesus as a messenger of god. But what sets Christianity apart is that they see Jesus as the son of God, and therefore, they see him as God. This verse encapsulates all that.