Which is the great commandment? - That is, the "greatest" commandment, or the one most important.
The Jews are said to have divided the law into "greater and smaller" commandments. Which was of the greatest importance they had not determined. Some held that it was the law respecting sacrifice; others, that respecting circumcision; others, that pertaining to washings and purifying, etc.
The law - The word "law" has a great variety of significations; it means, commonly, in the Bible, as it does here, "the law given by Moses," recorded in the first five books of the Bible.

Which is the great commandment - We see here three kinds of enemies and false accusers of Christ and his disciples; and three sorts of accusations brought against them.
1. The Herodians, or politicians and courtiers, who form their questions and accusations on the rights of the prince, and matters of state, Matthew 22:16.
2. The Sadducees, or libertines, who found theirs upon matters of religion, and articles of faith, which they did not credit, Matthew 22:23.
3. The Pharisees, lawyers, scribes, or Karaites, hypocritical pretenders to devotion, who found theirs on that vital and practical godliness (the love of God and man) of which they wished themselves to be thought the sole proprietors, Matthew 22:36.

Master, which is the great commandment in the law? He calls him "master, Rabbi, or doctor", as the Sadducees had in Matthew 22:24 either because he was usually so called by his disciples, and by the generality of the people; or merely in complaisance to engage his attention to him, and his question: and might hereby suggest, that should he return a proper and satisfactory answer to it he should be his master. The question is not which of the laws was the greatest, the oral, or the written law: the Jews give the preference to the law delivered by word of mouth; they prefer the traditions of the elders before the written law of Moses; See Gill on Matthew 15:2; but the question was about the written law of Moses; and not merely about the decalogue, or whether the commands of the first table were greater than those of the second, as was generally thought; or whether the affirmative precepts were not more to be regarded than negative ones, which was their commonly received opinion; but about the whole body of the law, moral and ceremonial, delivered by Moses: and not whether the ceremonial law was to be preferred to the moral, which they usually did; but what particular command there was in the whole law, which was greater than the rest: for as there were some commands that were light, and others that were weighty, a distinction often used by them (m), and to which Christ alludes in Matthew 23:23. It was moved that it might be said which was the greatest and weightiest of them all. Some thought the commandment of the sabbath was the greatest: hence they say (n), that he that keeps the sabbath is as if he kept the whole law: yea, they make the observance of the three meals, or feasts, which, according to the traditions of the elders, they were obliged to eat on the sabbath, to be at least one of the greatest of them,
"These three meals (says one of their writers (o)) are a great matter, for it is one , "of the great commandments in the law".
Which is the very phraseology used in this question. Others give the preference to circumcision, on which they bestow the greatest encomiums, and, among the rest (p), say, it drives away the sabbath, or that is obliged to give place unto it. Others (q) say of the "phylacteries", that the holiness of them is the greatest of all, and the command to be arrayed with them all the day, is more excellent than all others; and even of the fringe upon the borders of their garments, others observe (r), that a man that is guilty of that command, is guilty of all others, and that single precept is equal to all the rest. In this multiplicity of opinions, Christ's is desired on this subject, though with no good intention,
(m) Pirke Abot, c. 2. sect. 1. & c. 4. sect. 2. (n) Zohar in Exod. fol. 37. 1. (o) Tzeror Hammor, fol. 3. 3. (p) Misn. Nedarim, c. 3. sect. 11. (q) Maimon. Hilch. Tephillin, c. 4. sect. 25, 26. (r) T. Bab. Menachot, fol. 43. 2.

Which is the great commandment? This was a question which, with some others, divided the Jewish teachers into rival schools, and was a constant bone of contention--one of "those strivings about the law," against which Paul warns Titus (Titus 3:9). The Jews divided their commandments into greater and lesser, but were not agreed in particulars. Some pronounced the law of circumcision the greater; others, that of sacrifices, or ablutions, or phylacteries. The Talmud reckoned the positive laws of Moses at 248, the negative at 365, in all 613. To keep so many laws, said the Jews, is an angel's work. So they had much question which was the great commandment, so that they might keep it in lieu of keeping the whole.