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Unluckiest
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vilest
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wildest
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Xenophobicest
Okay, I don't think this word actually exists but it is grammatically correct, meaning "most xenophobic". To use it in a sentence, "He is the xenophobicest person I know", ie. "He is the most xenophobic person I know". I couldn't find a single word beginning with "X" and ending in "est" so I made up a grammatically correct word. Sorry.
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youngest
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zest
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Attest
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Briefest
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Craziest
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darkest