Jesus himself tells us what the greatest commandment is:
Matthew 22:35-40 Then one of them, which was a lawyer, asked him a question, tempting him, and saying, Master, which is the great commandment in the law? Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. This is the first and great commandment. and the second is like unto it, Thou shalt love thy neighbour as thyself. On these two commandments hang all the law and the prophets.
I hope you have found this helpful.
May God bless you,