It depends on what you call heavy: by size and moral weight, the strongest candidate is The Brothers Karamazov; by atmosphere, The House of the Dead; by political violence, Demons.
The Brothers Karamazov has twelve books plus an epilogue and brings together a parricide, a trial, and the debate over the suffering of children, which Ivan uses to refuse the world of God. Dostoevsky drew on details of a real trial, that of a couple accused of mistreating their five-year-old daughter, and in a letter to his editor he treated this argument as irrefutable.
Demons (1871-1872) starts from the murder of a student in Moscow by the revolutionary group led by Sergei Nechayev, in November 1869, and follows what ideas do to those who serve them. The House of the Dead, the author's own account of his years of penal servitude, shows a prison of peasants who hated the nobles.
Crime and Punishment is, by contrast, the most claustrophobic: a double murder, a fever, and the siege of an examining magistrate. None of them is light, and the weight comes less from size than from living alongside characters who carry an idea to its end.
On Clavis: Crime and Punishment.