Who is the greatest?

Solomon

I have given thee [Solomon] a wise and an understanding heart; so that there was none like thee before thee, neither after thee shall any arise like unto thee. 1 Kings 3:12

Jesus

John the Baptist talking about Jesus:

I indeed baptize you with water unto repentance. but he that cometh after me is mightier than I, whose shoes I am not worthy to bear. Matthew 3:11

Jesus talking about himself (saying he is greater than Abraham, Moses, David, Solomon, John the Baptist, and all the Hebrew prophets -- they, along with everyone who lived before him, were all a bunch of theives and robbers):

The queen of the south shall rise up in the judgment with this generation, and shall condemn it: for she came from the uttermost parts of the earth to hear the wisdom of Solomon; and, behold, a greater than Solomon is here. Matthew 12:42
The queen of the south shall rise up in the judgment with the men of this generation, and condemn them: for she came from the utmost parts of the earth to hear the wisdom of Solomon; and, behold, a greater than Solomon is here. Luke 11:31

The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which taketh away the sin of the world. This is he of whom I said, After me cometh a man which is preferred before me. John 1:29-30

All that ever came before me [Jesus] are thieves and robbers. John 10:8

John the Baptist

Jesus said that John the Baptist was the greatest person who was ever "born of women."

Among them that are born of women there hath not risen a greater than John the Baptist. Matthew 11:11
Among those that are born of women there is not a greater prophet than John the Baptist. Luke 7:28

Everyone in the kingdom of heaven.

John the Baptist was the best among people born of women, but everyone in the kingdom of heaven "is greater than he." [Since Jesus was born of a woman (Galatians 4:4), John the Bapist if greater than Jesus. And everyone in the kingdom of heaven is greater than John the Baptist and Jesus.]

Among them that are born of women there hath not risen a greater than John the Baptist: Among them that are born of women there hath not risen a greater than John the Baptist: notwithstanding he that is least in the kingdom of heaven is greater than he. Matthew 11:11
Among those that are born of women there is not a greater prophet than John the Baptist: but he that is least in the kingdom of God is greater than he. Luke 7:28

Whoever is as humble as a little child

At the same time came the disciples unto Jesus, saying, Who is the greatest in the kingdom of heaven? And Jesus called a little child unto him, and set him in the midst of them, And said, Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven. Whosoever therefore shall humble himself as this little child, the same is greatest in the kingdom of heaven. Matthew 18:1-4