For me, it was 'The Fault in Our Stars.' The way the characters had to face terminal illness and their love story was just heartbreaking.
I'd say 'A Thousand Splendid Suns.' The struggles and hardships the female characters endured throughout the story made it incredibly sad. The constant oppression and lack of hope really got to me.
One of the saddest was 'The Kite Runner.' The betrayal, guilt, and the long journey of redemption were emotionally overwhelming. The raw emotions and the harsh realities depicted made it a tearjerker.