The rarest Yugioh card ever has got to be Tyler the Great Warrior as there is only 1 in the universe!

Here is its description:

Tyler the Great Warrior
TYL-EN001
Warrior/Effect
EARTH
Level 8
ATK/3000 DEF/1500
This card cannot be Special Summoned. When this card destroys a monster as a result of battle and sends it to the Graveyard, inflict damage to your opponent's Life Points equal to the ATK of the destroyed monster.
Ultra Rare

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