There is no verdict: the best book by Dostoevsky depends on the criterion, and the most honest answer separates the criteria. As a synthesis and a point of arrival, The Brothers Karamazov; as a story and a point of entry, Crime and Punishment; as a rupture, Notes from Underground; as the riskiest bet, The Idiot.
The Brothers Karamazov was his last novel, published in serial form from early 1879 and completed in November 1880, shortly before the author's death. It brings together a family crime, the Grand Inquisitor, and Ivan's revolt against a world in which children suffer, and Dostoevsky wrote to his editor that he wanted to show the most extreme blasphemy and, with it, the refutation, in the final words of the elder Zosima. Crime and Punishment is the most direct reading: a crime at the beginning, a psychological siege, and a moral question that fits in one sentence. It was the book of the year in 1866.
Notes from Underground (1864) was cut by the censors precisely where it concluded that faith in Christ was necessary, and the author claimed the underground as his glory. In The Idiot (1868-1869), he tried to portray a man who was utterly beautiful, and he wrote to his niece that he had gambled, as at roulette. Frank calls it his most personal novel.
Two sensible readers may disagree: one prefers the novel that closes the work, another prefers the one in which idea and plot still move together. Dostoevsky himself left no ranking.
On Clavis: Crime and Punishment.