Readers decide that, but there are good reasons for it to be considered so: it is the author's last novel, serialized between 1879 and 1880, a few months before his death at the start of 1881, and it gathers in one book the themes he had worked on in the earlier ones.
The crime and remorse of Crime and Punishment return in Ivan and Mitya; the idea that becomes act, from Demons, is in the servant Smerdyakov; the holy and innocent man, from The Idiot, has his version in Alyosha. The Adolescent had already rehearsed the relationship between father and sons. And the consolation the author received from Elder Ambrose, at the Optina monastery, after the death of his son Alexei, who was almost three years old, became a scene in the novel.
Besides this, it has two blocks that can be read almost as separate works: the Grand Inquisitor and Ivan's rebellion, on one side, and the teachings of the elder, on the other, which answer them. The book is, above all, a debate between the two, fought in scenes and not in theses.
That does not make it the best starting point. It is long, has many characters, and read first it loses the resonance of the others. For someone who has already read Crime and Punishment and Demons, it is the reading that closes a path.