John 3.16 is one of the most famous and widely quoted verses from the Bible. Wikipedia notes that "it has also been called the 'Gospel in a nutshell', because it is considered a summary of the central theme of traditional Christianity."
The message is so powerful - it describes God's absolute love for us in that he was prepared to sacrifice his only Son for us. It also tells us that if we believe in Jesus we will receive eternal life.
As a Christian, I find this verse extremely moving as it summarises so well the cornerstone of our faith.