Is the greatest affair which Allah has commanded that of Tawhid?

I would disagree with this fundamental premise. It is incorrect.

The greatest affair is surely the imperative "think!" - why would we claim it was tawhid?

The incident in the cave hira at commencement of revelation is steeped in the necessity of reflection - from the first word, "iqra" to the incident itself which was confusing for the prophet(saw) asked him to read and reflect on the world around him.

The revelation continuously reminds us on the importance of reflection, consideration, reasoning, contemplation, understanding etc It is the core thread and that which was responsible for our rise and causal to our fall...
"It is a Book We have sent down to you, full of blessing, so let people of intelligence ponder (liyaddabbaru) its Signs and take heed." [Saad, 38:29]
"Will they not then ponder the Qur'an or are there locks upon their hearts?" [Muhammed, 47:24]
"Do they not look at the camels, how they are created? And at the heaven, how it is raised? And at the mountains, how they are rooted and fixed firm? And at the earth, how it is spread out?" [Ghashiyyah: 17-20]

The Prophet (saw) once came to the house of his wife A'isha (ra). In the depth of night, he made wudu and stood up in prayer wherein he was weeping so much that his beard became wet. Thereafter, he went to sujud and was crying so much that the ground became wet. He then laid down upon his side and continued to weep. As dawn approached, Bilal (ra) came to call for Fajr adhan when he came to alert the Prophet (saw) and saw him crying, whereupon Bilal (ra) asked:"O Prophet of Allah, why do you cry even though your past and present sins have been forgiven".The Prophet (saw) said, "Woe to you Bilal. Who will prevent me from crying? Allah has just tonight revealed this verse [Sura Al Imran: 190]. Woe to him who reads this verse and does not contemplate".

Other narrations mention "Groups of people will emerge from my Umma who will drink the Qur'an as they drink milk." (Tabarani)

In commenting on this hadith, Al-Munawi says "... that is, they will raise their voices with their tongues without contemplating and reflecting on its meanings and pondering over its rulings; instead it (i.e. the Qur'an) passes over their tongues as milk which they drink passes over them quickly."

Ibn Kathir commented on many of the verses in his tafsir saying
"These things should lead him [man] to see that He is the Lord, the Most Great, the Creator, the Owner and the Controller of everything. Therefore, He is Allah other than whom none deserves to be worshipped".

Thus reflection/contemplation etc precedes tawhid... it's a more fundamental and necessary act and I would say, far more important.

Genuine tawhid begins after the cessation of thinking. Indeed, genuine reflection begins after the end of thinking. But who can escape the endless deluge of thoughts?

We don't think thoughts; they come to us. The beginning of the West's afflictions (not necessarily the cause though) is Descartes "I think therefore I am".

without wishing to trivialise an issue that is certainly challenging, our ideas of our self-form and our identity are little better than crude approximations built by us to fit the conditioning we received as children. We do not have an adult sense of identity. Rather we are propelled out of our families into the world with makeshift senses of self and programmed with certain ambitions and desires, sometimes to fulfil our parents' unfulfilled ambitions - the poet said: If I don't make it, perhaps my baby will - and then we play in the world, often to disastrous effect.

As for intellectuals we play with ideas. Given a certain IQ, we can easily manipulate ideas and receive the acknowledgement of other 'intellectuals' of similar disposition. But there is nothing serious in this and it is not even the beginning of intellect because there is no self-knowledge in it at all. The first command is "Know thyself".

The point is whether contemplation was the starting point of the call of all Prophets. The Qur'an is replete with examples of the Prophets' call and in most, if not all, cases there is no mention of contemplation before Tawhid. Let us consider the following verses:

1. 16:36 And We certainly sent into every nation a messenger, [saying], "Worship Allah and avoid Taghut." And among them were those whom Allah guided and among them were those upon whom error was [deservedly] decreed. So proceed through the earth and observe how was the end of the deniers.

2. 21:25 And We sent not before you any messenger except that We revealed to him that, "There is no deity except Me, so worship Me."

3. 21:92
Indeed this, your religion, is one religion and I am your Lord, so worship Me.

4. 23:51-52
[ Allah said], "O messengers, eat from the good foods and work righteousness. Indeed, I, of what you do, am Knowing.
And indeed this, your religion, is one religion and I am your Lord, so fear Me."

5. 11-25/26
And We had certainly sent Noah to his people, [saying], " Indeed, I am to you a clear warner. That you not worship except Allah . Indeed, I fear for you the punishment of a painful day."

so on and so forth.

The only example that supports your viewpoint somehow is the story of Abraham Alaihi salat but that too is actually calling to Tawhid.

Thinking which the Qur'an recommends is not noncategorical; it is bound to caveats and restrictions. It is a slave to Creed and Belief. Independent and Free Thinking per se is NOT a Qur'anic endorsement, Thinking which somehow helps to confirm Religion through a dose of Rationality is. It is FAITH the highest Qur'anic Virtue, not mere Thinking.

There is naught which warrants to conclude that the pith and marrow of the Prophets' Call was Thinking per se. The Messengers rather appealed to people's emotions and instincts and asked them to simply trust them as the receivers of Revelation and so get rewarded in the AfterLife in return, thus inaugurating their movements by laying the foundation through Creed and Belief. It is this, in fact, which is so bulgingly evident across all of the Qur'an by means of Repetitive Emphasis ("Istiqrar").

Hood, 25-27:
And We sent Noah unto his folk (and he said): I am a plain warner unto you that ye serve none, save Allah. Lo! I fear for you the retribution of a painful Day. The chieftains of his folk, who disbelieved, said: We see thee but a mortal like us and we see not that any follow thee save the most abject among us, without reflection. We behold in you no merit above us, nay, we deem you liars.

Maryam, 41-50:
And make mention in the Scripture of Abraham. Lo! he was a saint, a Prophet. When he said unto his father: O my father! Why worshippest thou that which beareth not nor seeth, nor can in aught avail thee? O my father! Lo! there hath come unto me of knowledge that which came not unto thee. So follow me and I will lead thee on a right path. O my father! Serve not the devil. Lo! the devil is a rebel unto the Beneficent. O my father! Lo! I fear lest a punishment from the Beneficent overtake thee so that thou become a comrade of the devil. He said: Rejectest thou my gods, O Abraham? If thou cease not, I shall surely stone thee. Depart from me a long while! He said: Peace be unto thee! I shall ask forgiveness of my Lord for thee. Lo! He was ever gracious unto me. I shall withdraw from you and that unto which ye pray beside Allah and I shall pray unto my Lord. It may be that, in prayer unto my Lord, I shall not be unblest. So, when he had withdrawn from them and that which they were worshipping beside Allah, We gave him Isaac and Jacob. Each of them We made a Prophet. And We gave them of Our mercy and assigned to them a high and true renown.

Yusuf, 37-40:
Lo! I have forsaken the religion of folk who believe not in Allah and are disbelievers in the Hereafter. And I have followed the religion of my fathers, Abraham and Isaac and Jacob. It never was for us to attribute aught as partner to Allah. This is of the bounty of Allah unto us and unto mankind; but most men give not thanks. O my two fellow prisoners! Are divers lords better, or Allah the One, the Almighty? Those whom ye worship beside Him are but names which ye have named, ye and your fathers. Allah hath revealed no sanction for them. The decision rests with Allah only, Who hath commanded you that ye worship none save Him. This is the right religion, but most men know not.

Zumur, 11-14:
Say: Lo! I am commanded to worship Allah, making religion pure for Him (only). And I am commanded to be the first of those who surrender (unto Him). Say: Lo! if I should disobey my Lord, I fear the doom of a tremendous Day. Say: Allah I worship, making my religion pure for Him (only).

Araaf, 158:
Say: O mankind! Lo! I am the messenger of Allah to you all, Him unto whom belongeth the Sovereignty of the heavens and the earth. There is no God save Him. He quickeneth and He giveth death. So believe in Allah and His messenger, the Prophet who can neither read nor write, who believeth in Allah and in His words and follow him that haply ye may be led aright.

Hence, I will reiterate that it is FAITH-BASED TAWHID the highest of Qur'anic Virtues and NOT mere Thinking. Whenever the Qur'an asks "Aafala Ta'qilun?" it is almost always preceded by, or post-fixed by, a grand doctrinal description of Allah's Godhead and Greatness, which primes and enslaves the reader's mind and attention and mode of logic. Thus the kind of Thinking which the Qur'an recommends is implicitly, if not explicitly, restrictive and exclusive. It is not what is called Independent, Original, Free or Critical Thinking whose very heart-beat is skepticism and rejection and which nourishes on and progresses through trial and error.

The address of revelation is to all humanity and not specific to the prophet(saw).

The first command to the Prophet (saw) was to read, to interpret. The Prophet asked, "How should I read?" The angel replied, "Read, read in the Name of God..."

The key to understanding the cosmos is to read it like a book.

There are only two ways to read this book: self-referential or other-indicative.

To read a book in a self-referential way is to interpret it as a collection of phenomena which point only to themselves and nothing more.

The Other-indicative way to read is to interpret it as a collection of signs, each of which points to the Names and Attributes of its Author. Everyone reads the Divine book of the cosmos, but it is only reading which is done 'in the name of God' which yields the truth. Thinking, deliberating and rationalising (tafakkur, tadabbur and ta'aqqul) are enjoined on us by the Qur'an and can be understood as the means whereby "Iqra!" takes place.

The command 'Read!' applies to everyone, for we are all recipients of revelation.

True belief - investigative rather than imitative belief - is impossible without knowledge, which requires pondering. What one does with the knowledge one gains is an issue of will.