Hamlet is a flawed character. Which of his flaws can be considered the tragic flaw that leads to his downfall?

Hamlet's most resonant flaw is his reluctance to take action.  Though his father's ghost visits him and exhorts him to avenge his death, Hamlet's indecisiveness ultimately enables Claudius to get the upper hand; as a result, Hamlet and his mother die (as does his corrupt uncle, Claudius, and Laertes, Ophelia's brother).  It can also be said that Hamlet's inability to act leads to Ophelia's suicide.

Hamlet fails to act in several situations which ultimately results...

Hamlet's most resonant flaw is his reluctance to take action.  Though his father's ghost visits him and exhorts him to avenge his death, Hamlet's indecisiveness ultimately enables Claudius to get the upper hand; as a result, Hamlet and his mother die (as does his corrupt uncle, Claudius, and Laertes, Ophelia's brother).  It can also be said that Hamlet's inability to act leads to Ophelia's suicide.

Hamlet fails to act in several situations which ultimately results in a cascade of tragedies.  He could decide to take Ophelia into his confidence instead of acting mad and rejecting her.  He could tell his mother explicitly that he knows the circumstances of his father's death. He could confront Claudius directly and win back the throne of Denmark, since it is rightfully his.  He could kill Claudius when he finds him defenseless and at prayer.  However, because Hamlet is crippled by indecision and an inability to recognize whom he can trust, he and others die, and the throne of Denmark passes outside Hamlet's family.