The question pertains to the nature of prophecy itself.
First, Most prophecy of Messiah was intended as not fully understandable, until it takes place. This prevents anyone pretending to do it. This is why prophecy is often in visions, to ensure it is not clear. Prophecy therefore, in part, was set up as an incomprehensible riddle, encrypted with a hidden key. A key that no one could find, like a scroll of history, whether past, present, or future, all was put behind a veil. The scroll was rolled up and sealed with no one worthy to open it.
However Christ, by His work on the cross is the great collector, descriptor and explainer of all prophecies, which were otherwise ‘scattered in details’ leading to as many questions as answers. He was the great decoder key, where instantly all the prophecies became as a collection, heavenly convincing fireworks. He also was given authority to break the seals of future revelation. (Rev Ch 5).
Therefore although there were many details in the prophecies about Messiah, like where He would be born, through what genealogy, how He would restore the church that He would suffer, that He would usher in the end of the world, etc., the Jews were not able to fully comprehend the overall picture this presented. Once can see ho Christ’s fulfillment of that collected picture was reprehensible to them, while not disputing on any of the specifics.
For some understanding of what Messiah the Jews expected see ths post.
Second, many of the prophecies fulfilled by Christ were prophetic themes, rather than mere details surrounding those themes. For example the priesthood, the kingship, the prophetic offices were all Bible themes that run up into Christ as the great Prophet, Priest and King. Even the Rabbis knew this. In fact the Rabbinic Talmud, "All the prophets prophesied not but of the days of the Messiah" said (כל הנביאים כולן לא נתנבאו אלא לימות המשיח Sanh. 99a).
With this in mind there are several candidates for the most convincing prophecy. One could choose the most ‘descriptive’ prophecy such as Isaiah 53, which was understood as Messianic by the Rabbis. One could say that the promise to Abraham Gen 12:2 whereby ‘many nations’ would be blessed was the greatest. Later this son of Eve was limited to the seed of David. The reason that this is so convincing is that after the destruction of Jerusalem, along with all the genealogical records, to ever prove a Hebrew religious leader was born of David became impossible. In addition, how rare is it for ‘many nations’ to receive a religion originating from Israel. How else could someone born of David become a religious blessing to other nations? Only one man has been born of David and has been received by gentile nations, Jesus.
Along with the most descriptive and the most impossible to repeat, any prophecy that was ‘great’ must be one that speaks about his incarnation, death or resurrection. For His incarnation, “The virgin will be with child and will give birth to a son” seems quite rare and difficult to repeat! (Math 1:23) With respect to his death and resurrection, our Lord himself said to a companion who was trying to stop his arrest:
52“Put your sword back in its place,” Jesus said to him, “for all who draw the sword will die by the sword. 53Do you think I cannot call on my Father, and he will at once put at my disposal more than twelve legions of angels? 54But how then would the Scriptures be fulfilled that say it must happen in this way?”
So what prophecy was Jesus referring to here? Certainly this prophecy must be great? Yet I do not think it is a single prophecy, but in some ways all prophecy. The ceremonies practiced by the church, where each day animals were killed to atone for the sins of the people were prophecy. Jonah in the belly of a whale for three days only to be received by a gentile nation was prophecy. Every history in the Bible, every prophet, every ceremony, all ran up into prophetic themes satisfied in Christ!
There a different groups of prophecy. Some focus on Christ as a King, some as a suffering Servant, some as a unknown humble Branch, some as the killer of sin and the Devil, some refer to his required genealogy, some refer to the timing of his birth, some as casting light unto the Gentiles whereby they enter into the blessing of Abraham, some spoke of the great power of the Spirit in His ministry, some are highly apocalyptic and even reach into the days at the end of the world, some speak of the way he kept his enemies in the dark while teaching those who had ears to hear, yet all are explained and satisfied by Christ. Yet if we remove Christ they all become riddles once again, creating more questions than answers.
Therefore the fact that only one man, Jesus the Christ can integrate, and explain all of the Bible and that no man can or ever could attempt to do such a thing proves the New testament to angels, demons, saints, sinners and any who have a mind to reason.