If you only have time to read one text, pick the Mahabharata

The Mahabharata is many things to many people.

Madhvacharya's Mahabharata Taatparya Nirnaya, states that the Mahabharata was called "Jaya", for it outweighed the Vedas and other Shastras, as determined in a congregation of rishis post-Mahabharata. On a more simplistic level, "Jaya" is usually said to refer to the Pandavas' (dharma's) victory over evil.

The primary message of the Mahabharata is that of Vishnu's supremacy and independence. This may not be readily apparent to the casual reader, who might treat it more as a war among cousins and their cronies for control of land. To this end, here are summaries of a few episodes...

Krishna comes to the rescue many a time.

The episode that most comes to mind is that of the Draupadi Vastraapaharana. When even the mighty Bhima is unable to do anything, Krishna is there to help at a mere prayer from Draupadi to save her modesty. Here, rather than infer that Bhima is powerless (which he is not really, there are other factors in that episode like his own brother's commitment), one must infer that Bhima and Draupadi, both, pray to Krishna for help - for they are dependent on Him, the supreme being.

The Durvasa-Draupadi Episode
In another episode during the 12 year banishment to the forest, the Pandavas eat good food (and are able to feed the rishis who accompany them) using an akshaya paatra from Surya. The akshaya paatra would only produce food until Draupadi, the lady of the house, had eaten. Meanwhile, Rishi Durvasa visits Duryodhana's palace in Hastinapura, and pleased with his service, grants him a wish. Duryodhana uses the opportunity to request that Durvasa and his group of rishis visit the Pandavas in the forest, knowing well that Durvasa would curse the Pandavas when they are unable to feed him and his group as befits a king. (Durvasa was know to have a short temper. He appears earlier in the Mahabharata when Kunti as a young princess serves him well and receives the boon of invoking any God for having sons).

By the time Durvasa arrives at the Pandava camp, Draupadi has already eaten and the akshaya paatra has stopped producing food for the day. Yudhishtira welcomes the rishi, directs them to the river for a bath, and asks Draupadi to prepare to serve the rishi and his group. Draupadi, who is cornered, prays to Krishna to save her and her family from Durvasa's curse. Krishna appears at a moment's notice, asks to be shown to the akshaya paatra, and eats a morsel that remained. With this, he tells Draupadi to wait and watch. When Durvasa and his group return from the river, they tell Yudhishtira that they are full, and that they would have to continue on their way. Krishna uses the single morsel of food as a token offering from his devotee, and saves her from doom yet again.

The Caveat to 'Yada Yada hi Dharmasya Glaanihi bhavati... Tadaatmaanam Srujamyaham'
There is an important caveat to this statement from Krishna. Dharma being in peril is intrisically tied to the goodness of people.

• In the Krita Yuga, He appears as Matsya (to save the world from complete extinction), as Koorma (to help the devas win against the asuras), as Varaha (to retrieve the earth hijacked by Hiranyaksha), as Vaamana (again to help the devas win against Bali, an asura by pushing him down to a Paatala loka).
• It is only the Nrsimha avatara that is dedicated to one single person's devotion - Prahlada. Prahlada's father Hiranyaksha had declared himself God, and was obstructing all real worship. However, it is Prahlada's unwavering devotion and stance that makes the Nrsimha avatara happen.
• In the Treta Yuga, Parashurama and Rama avataras happen; the Parashurama avatara to generally rid the earth of several tyrannical rules right up to the Dvapara Yuga.
• In the Dvapara Yuga, Krishna takes avatara to rid the world of several rakshasas and helps the Pandavas rid the world of the Kauravas.

While the conditions in the Kali Yuga are easily much worse than in the previous yugas, why does He not incarnate? He does not because there is none that upholds the Dharma truly and completely as do his devotees who he immediately comes to help (in person) in the previous yugas. We are all complicit to varying degrees in the adharma that fills the world today, and the next avatara according to the Puranas is the Kalki avatara which will end the world and usher in the next mahayuga. That is not to say that there are no good people in Kali Yuga; there are. Just not good enough to merit His avatara. He still guides and shadows the deserving and undeserving alike, unseen, to provide them the best paths to their destiny.

Kunti's Parting Request to Krishna
When the war is over, and Krishna seeks to return to Dwaraka, Kunti requests him to give her sorrow. For, it is only in sorrow that we remember Him. This, after all the sorrow that she and her sons had faced. Kunti's first major blow was that of the birth of Karna via Surya. She then loses her husband, and has to raise 5 children. The 5 sons, powerful and divine though they might be, go through a series of attacks - hidden and direct - by the Kauravas, including the gambling, banishment, and ultimately the war. Kunti thanks Krishna for his hand in saving her family from all the pitfalls and dangers, instead of asking for a life of happiness going forward. She truly recognizes that happiness on earth is not eternal, and that the opportunity to know Him more through contemplation is worth more than happiness, to help in her path to eternal bliss, Moksha.

Krishna in the War
Before, during, and after the war, the Pandavas will not have been able to accomplish anything without Krishna. Before the war, it is with Krishna's guidance that Bhima is able to destroy Jarasandha. It is with Krishna's guidance that the Pandavas win and marry Draupadi. It is with His guidance that they are able to overcome Bhishma, Drona, and Ashwattama. While Bhima may have been able to win the war single-handedly, the other brothers needed His help to even survive. Krishna saves Arjuna in more than one occasion (helps win against Karna by provoking him to waste his Shakti astra against Ghatotkacha, for example). If there is one truly revealing spot in the Mahabharata where even Bhima's dependence on the supreme being, it is when Ashwattama uses the Narayanaastra against the Pandavas. Krishna instructs the Pandavas, including Bhima, to lay down their arms and surrender to the astra completely to be left untouched.

• Bhima, who as his previous incarnation Hanuman, is unaffected by even the Brahmaastra (he only surrenders to it out of respect) can only be defeated using the Narayanaastra. Bhima, unknowingly fights the astra (likely urged by Krishna to do so, for Bhima would not fight the only astra superior to him on purpose) and successfully holds against it, but is worn down over time. Finally, when he is told by his brothers that it was the Narayanaastra and to surrender, he does so. I believe the only purpose of this episode was to highlight Bhima as the foremost among the jeevas and devas.   

Krishna keeps the Kuru Dynasty Alive

Krishna saves Parikshit (in the womb of Uttara, the dead Abhimanyu's wife) from the Brahmaastra of Ashwattama. Krishna stops it using his chakra, to retain the Pandavas' last remaining prince to keep the Kuru dynasty alive. 

There is no doubt, from these examples that Vishnu's (Krishna) supremacy, and his complete support of his true devotees, is the primary message of the Mahabharata. In addition, since it encompasses the messages of all other shaastra texts combined as well as all possible topics under the sun, worldly and spiritual, it is one that we can safely hold on to for our intellectual upliftment.