Matthew 22:36 Master, which is the greatest commandment in the law? 37 Jesus said to him: Thou shalt love the Lord thy God with thy whole heart, and with thy whole soul, and with thy whole mind. 38 This is the greatest and the first commandment. 39 And the second is like to this: Thou shalt love thy neighbour as thyself. 40 On these two commandments dependeth the whole law and the prophets.
Seems like that would be the greatest commandment. Everything else depends on the two.
In fact, God says that you will not enter the kingdom of God if you do not do His will. In Matthew 25 He says that what you do for the least among us, will determine where you spend eternity. He also says that some of the saved will not know why they are being saved, which means they aren't even believers. But what they did saved them.
Matthew 25:37
Then shall the righteous answer him, saying, Lord, when saw we thee an hungred, and fed thee? or thirsty, and gave thee drink? 38 When saw we thee a stranger, and took thee in? or naked, and clothed thee? 39 Or when saw we thee sick, or in prison, and came unto thee? 40 And the King shall answer and say unto them, Verily I say unto you, Inasmuch as ye have done it unto one of the least of these my brethren, ye have done it unto me.