The Winged Dragon of Ra is basically the strongest because while Obelisk and Slifer have a fixed amount of attack points, the Winged Dragon of Ra has infinite.
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the winged dragon of ra depending on usage
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No part of Winged Dragon of Ra's effect stops you from Special
Summoning while you have him out on the field.
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The Winged Dragon of Ra is it's at The Final Boss card shop and
maybe Slifer as well