The opinion of the Rambam is that while the resurrection of the dead is one of the fundamental principles of the Torah, the ultimate goal is life in the World to Come. After the resurrection, people will live a long life but will eventually die again. Their souls will live eternally in the World to Come, which is entirely spiritual.

Life after the resurrection of the dead is not eternal:

The resurrection of the dead is a fundamental principle of the Torah, and one who does not believe in it has no portion in the Torah of Moses our teacher. But it is not the ultimate goal. That goal is the World to Come…. Those people whose souls will return to their bodies will eat, drink, engage in sexual relations, and have children, yet they will die after a very long life, like the lifespans of those living in the messianic era. The life that is not followed by death is the life of the World to Come, since that is a life without a body.

The messianic era will last for a long time, the anointed king will die, and his son will rule after him:

The anointed king (the messiah) will die, and his son will rule after him, and then his grandson. The prophet already told of the death of the anointed king: “He will not weaken and will not be broken until he establishes justice in the land” (Isaiah 42:04). His kingdom will last for an exceedingly long time, and people’s lifespans will also be very long, because when worries and sorrows will be removed, people’s lifespans will increase. Do not be surprised if this kingdom will last thousands of years, for when a group of good people bands together it does not quickly disband. (Rambam, Igeret Teĥiat HaMetim; Commentary on the Mishna, Introduction to Ĥelek)

The opinion of the Ramban is that after a person’s death, the soul enters a spiritual world referred to as the Garden of Eden. Eventually, the messianic era will arrive, which will be an epoch in this world. Finally, the time for the resurrection of the dead will arrive. This is the time of the ultimate reward, and is referred to by the Sages as the World to Come.

After the dead are revived they will not die again:

We will therefore believe what [the Sages] said: “The dead that the Holy One, blessed be He, will, in the future, revive will not return again to their dust” (Sanhedrin 92b)…. This matter is often repeated by the Sages, as they state in many places in the Midrash and Talmud: “The expression: ‘That it may be well with you,’ is a reference to the world where all is well, and the expression: ‘That your days may be long,’ is a reference to the world that is entirely long” (Kiddushin 39b). Their intent is that all who merit [to be resurrected] will have long life, for there will be no death.

The body will be like the soul and will never die again:

The place of reward of the souls and their continued existence in the world of souls is called by our Sages “the Garden of Eden.” They sometimes refer to it as “ascension” or “the heavenly academy.” Later, the messianic era will arrive, which will take place in this world. Finally, there will be the day of judgment and the resurrection of the dead, which is a reward that includes both body and soul. That is the great, primary [goal] for which all who place their hope in the Holy One, blessed be He, aspire, and that is the World to Come, where the body will become like the soul, and the soul will cling to the knowledge of the Most High, as it did when in the Garden of Eden, the world of souls. [The soul] will be elevated with even greater spiritual comprehension, and all of this will continue to exist eternally. (Ramban, Torat HaAdam, Sha’ar HaGemul)

The goal of the creation of the world and of humanity is for this dark, lowly world to become a place where God’s presence dwells and is openly apparent. Consequently, the final and lasting reward is specifically in this world.

God desired that His dwelling place be in the lower levels of existence:

The reason for the creation of the world in general and the creation of humanity in particular, and their purpose, is that the Holy One, blessed be He, desired a dwelling place specifically in the lower spheres. Meaning, the lower spheres…[which include all of] physical existence, should be in a state of negation [relative to God]. In this manner, the light of the infinite God, may He be blessed, will rest on them and be revealed through them…. This is why the Torah and commandments were given below and apply to physical entities and to a soul resting specifically within a physical body. This is so their performance can transform independent existence into negation, making it a vehicle for Godliness.

Man’s perfected state will come through the resurrection of the dead, when his soul will rest within a physical body:

In the world after the resurrection, God will totally remove the spirit of impurity from the world; there will no longer be sin or death in the world. For the Holy One, blessed be He, will slaughter the evil inclination, which is also the angel of death. Humankind will then reach its absolute perfection, not only as a result of its [divine] service and the reward for it, but also through a gift given from above…. This reward will be received below specifically by a soul that is in a body. That will be the ultimate purpose and goal of this world, for the world was originally created so that God, may He be blessed, will have a dwelling place in the lower spheres. (Rabbi Menaĥem Mendel Schneerson, Igrot Kodesh 2:200)

Further reading: For more on the idea that the goal of the physical world is to serve as an abode for God’s presence, see p. 146.