The most powerful shield according to Hindu texts. I will mention the top 3 most powerful shields (armors) in Hinduism and Hindu texts.
The most powerful shield according to Hindu texts
1). SHATACHANDRA SHIELD (Of Supreme Lord Vishnu)
The Shatachandra Shield of Supreme Lord Narayana is the strongest among all the shields in Hinduism.
Vishnu’s swift club, known as Koumadaki, the sword Vidyadhara, the shield Shatachandra and two inexhaustible quivers filled with arrows. Sunanda and the other foremost attendants, along with the guardians of the world, waited upon the Lord. His headdress, armlets, and earrings shaped like fish dazzled.
Srimad Bhagavatham, Canto 8, Chapter 20
Lord Vishnu has slain 1000 billion demons within a Muhurata (48 minutes) using this shield.
Hari shook them off and caused them to fall on the ground like an enlightened man shaking off all the blemishes from worldly existence. Viṣṇu drew out his excellent sword Nandaka from its sheath and took up the shining stainless shield. He rushed after them on foot. Within a Muhūrta (=48 minutes), Keśava cut off on the way, one thousand billion excellent daityas after moving about many times (on the battlefield).
Skanda Mahapurana, Maheshwara-Khanda, Kaumarika-Khanda, Chapter 20
2). KARNA’S KAVACHA-KUNDALA (Gifted by Lord Surya)
Karna had the celestial armor and earrings attached to his body by birth.
Hearing her words, Surya answered, ‘O gentle maiden, thy son shall be mighty-armed and decked with earrings and a celestial coat of mail. And both his earrings and coat of mail will be made of Amrita, and his coat will also be invulnerable.
As Lord Surya said, celestial armor and earrings were made up of Amrita, and we’re impenetrable.
Kunti then said, ‘If the excellent mail and earrings of the son thou wilt beget on me, be, indeed, made of Amrita, then, O God, O worshipful deity, let thy purpose be fulfilled! May he be powerful, strong, energetic, and handsome, even like you, and be endued with virtue!’
Lord Surya gave those Kavach Kundal to Karna, offered to Lord Surya by Aditi.
Surya said, ‘O princess, O excellent damsel, these earrings had been given to me by Aditi. O timid lady, I will bestow them, as also this excellent mail, on thy son!’ Kunti then said, ‘Very well, O worshipful one! If my son, O Lord of light, become so, I will, as thou sayest, gratify thee!'”
Mahabharata, Vana Parva, Pativrata-Mahatmya Parva, Chapter CCCV
According to Sri Krishna, neither Arjuna with Gandiva nor Sri Krishna with Sudarshana Chakra could break the armor or slay Karna.
Indeed, if that bull among men(Karna) had his armor and earrings, neither thyself, bending the Gandiva, nor myself, uplifting my discus, called Sudarsana, could defeat him in battle.
Mahabharata, Drona Parva, Ghatotakacha vadha Parva, Chapter CLXXX
3). BRAHMA KAVACHA OF RAVANA (Given by Lord Brahma).
Lord Brahma gave the divine armor to Ravana.
“As a consequence of that austerity alone and by the grace of Brahma the lord of creation, there was never a fear for me either from the demons or from the celestials.”
“In combats with the gods and demons, the armor gifted to me by Brahma, the lord of creation, which is as brilliant as the sun, could not be mutilated by those who held the thunderbolt in their fists.”
Ravana survived the Vishnu Chakra hundreds of times due to the armor of Lord Brahma.
His body bears hundreds of welts caused when assaulted with Vishnu-disc hundreds of times in wars with gods, and when attacked hundreds of times with the other assaults-missiles in other gruesome battles as well, and Shurpanakha saw at such a Ravana.
Due to this armor, multiple Raudrastras of Sri Rama could not even harm Ravana.
Thereupon, seizing hold of more arrows, reciting sacred chants, and using the missile presided over by Rudra and stretching his bow, the valiant Rama of great splendor was filled with anger and released those arrows.
Fallen on the unbreakable armor of Ravana, which looked like a giant cloud, those arrows could not cause any pain to him at that time.
This is what Sri Rama thought when his arrows could not kill Ravana.
“What is the reason, these arrows by which Mareecha, Khara, Dushana, Kabandha in Kroucha-forest, and Viradha in Dandaka-forest were killed, by which seven Sala trees and the mountains were burst, by which Vali was killed and the ocean shaken up all these arrows which provided immediate succor to me in battle, have proved of little efficacy in the case of Ravana.”
Honorable mentions:-
SHIVA-KAVACHA:-
Shiva-Kavach: The armor of Lord Shiva which will make its wielder invincible.
JAIVARDHANA KAVACHA:-
JaiVardhan Dhaal: A shield of Lord Vishnu and Lord Shiva.
SRIVATSA KAVACHA:-
Sivatsa Kavacha: A shield presided over by Lord Vishnu. It is the most different kind of shield (armor). It is a symbol considered the shield of Lord Vishnu and is often manifested on his chest.