Question: Which case was heard by largest ever constitution bench by 13 supreme court judges?
Answer: His Holiness Kesavananda Bharati Sripadagalvaru and Ors. v. State of Kerala and Anr.
Kesavananda Bharati v. State of Kerala
Natural Questions answer: Golaknath v. State of Punjab (from the Wikipedia article "Kesavananda Bharati v. State of Kerala")