If we go by the epic, there is no single, definitive "chosen one." Rather, there are multiple chosen ones depending on the definition and criteria being used. In my opinion, the main chosen ones are Karna, Lord Krishna, Arjuna and partly Duryodhana (he's more like a supporting character).
For example, if the criterion is whose destiny was to create dissension within the Kuru lineage and ultimately lead to the Great War, then I would argue it is Karna. I don't think Duryodhana fits this criterion, for the simple reason that it was only after Karna entered the picture to challenge Arjuna and the Pandavas that Duryodhana started actively pursuing schemes to destroy the Pandavas and remove them from the equation. Before that, he generally tolerated Bhima in public and only attempted to kill him secretly by poisoning and drowning him. He never sought a great war, nor did he even seem to think about one. Instead, he relied on petty methods such as sending the Pandavas away, attempting to kill them secretly in the Laksha Griha while making it look like an accident, or defeating them indirectly through gambling and exile. Duryodhana never intended to openly wage a great war to destroy the Pandavas.
It was Karna who repeatedly instigated war and deeper dissension. After Draupadi's svayamvara, he advised Duryodhana to immediately attack the Panchalas and the Pandavas before they could gather allies. During the Virata war, he urged the Kauravas to kill Arjuna quickly so that the remaining Pandavas would commit suicide. Throughout the epic, it is almost always Karna who instigates greater dissension between the Kauravas and the Pandavas and pushes for war. This role is repeatedly highlighted by Narada, Bhishma, Drona, and Vidura.
By the same reasoning, I don't think even Lord Krishna fits this specific criterion of creating war and dissension within the Kuru dynasty. His purpose is slightly different. His mission is to destroy evil, reduce adharma, and lighten the burden of the Earth by eliminating wicked rulers. He did not require the Kurukshetra War to accomplish that. He had already slain many evil kings and beings such as Kamsa, Narakasura, Shishupala, Putana, Bakasura, Shalva, Banasura, and many others. Jarasandha, too, was eliminated before the Kurukshetra War through Krishna's own plan. In Ghatotkacha vadha Parva, after the death of Ghatotkacha, Lord Krishna himself tells Arjuna that, if necessary, these evil rulers would have perished regardless.
Krishna also made repeated attempts to prevent the war. He negotiated for half the kingdom, then for five villages. He personally travelled to Hastinapura as a peace envoy, revealed his Vishvarupa before Duryodhana and Karna, and even suggested arresting—or, if absolutely necessary, killing—Duryodhana for the welfare of the kingdom. Every attempt at peace failed. So, Krishna is the chosen one in the sense that he was destined to destroy evil and reduce the burden of the Earth.
In the same way, Arjuna is the chosen one in the sense that he serves as Krishna's divine instrument for eliminating that evil. He destroys or helps destroy many of the principal obstacles, including the Nivatakavachas, Kalakeyas, Bhishma (through Shikhandi), Karna, and numerous Naga warriors. Throughout the epic, Arjuna is repeatedly protected and empowered by divine intervention. Krishna protects him from Karna's Vasavi Shakti, the serpent-headed weapon, the Brahmastra, Drona's celestial weapons, Ashwatthama's Agneyastra, and, through divine grace and Lord Shiva's boon, the Narayanastra.
This is also why, after Krishna stepped down from Arjuna's chariot at the end of the war, the chariot immediately burst into flames. Later, after Krishna departed from the Earth, Arjuna could no longer effectively wield the Gandiva or even remember many of his celestial weapons. As a result, he was defeated by the Abhira robbers and failed to protect Krishna's wives.
Along with this, Arjuna was also aided by various divine beings on different occasions. During his battle against the Saindhava forces, when he was nearly killed and rendered unconscious, the Brahmarshis revived him and restored his energy. Hanuman sat upon his chariot, weakening Arjuna's enemies through his mighty roars while protecting both Arjuna and his chariot from numerous celestial weapons. Lord Shiva likewise aided him during the fourteenth and fifteenth days of the Kurukshetra War. Arjuna was repeatedly destined, protected, and empowered to accomplish the tasks ordained for him by the gods. Thus, Arjuna can indeed be regarded as the chosen one in the sense that he serves as the divine instrument through whom much of that destiny was fulfilled.
Just like Arjuna serves as Lord Krishna's divine instrument, I would consider Duryodhana to be an aid or a means through which Karna's destiny of bringing about a great war is realised. That is why it was only after Karna entered the picture that Duryodhana began openly opposing the Pandavas and actively turned against them. It was Karna who constantly fuelled their enmity, whether during the Ghosha Yatra Parva or the Virata Parva. It was only with Karna's support and encouragement that Duryodhana became willing to openly confront the Pandavas in battle.
By himself, Duryodhana often appears too insecure and politically cautious to directly wage war against the Pandavas. Instead, he preferred indirect methods such as poisoning Bhima, the Laksha Griha plot, and the game of dice. A good example is the Ghosha Yatra episode, where, after being humiliated and rescued by the very Pandavas he hated, Duryodhana was prepared to give up his life out of shame. It was Karna who, through his persuasive words, restored Duryodhana's resolve. Even after that, it was only when the demons assured him of their support and reminded him of his purpose that he regained the will to live and continue his struggle.
So, in that sense, Duryodhana can be seen more as an instrument through whom Karna—and, by extension, the demons' objective of bringing about the great conflict—fulfilled their purpose, much like Arjuna serves as Lord Krishna's and the devas' instrument in accomplishing theirs.
There are also what could be called "mini chosen ones"—characters whose destinies are tied to accomplishing a specific purpose rather than the overarching narrative. For example, Draupadi is often described as one of the reasons behind the Great War, although I would argue that she was only one contributing factor and that the struggle for the Kuru kingdom was the primary cause. Similarly, Dhrishtadyumna was born specifically to bring about Drona's death, while Shikhandi's destiny was inseparably linked to Bhishma's fall. In the same way, many other characters in the epic were "chosen" for particular roles or destinies. Thus, rather than having one exclusive chosen one, the Mahabharata presents multiple chosen figures, each fulfilling a different divine purpose depending on the context and the criterion being considered.