From the Denver Post (for details, follow the link; all descriptions deleted for space and to follow copyright guidelines):
The best April Fools' hoaxes of all time have been ranked by the San Diego-based Museum of Hoaxes.
The list rates pranks based on notoriety, absurdity and the number of people fooled.
Here they are, in order:
1: The Swiss Spaghetti Harvest
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#2: Sidd Finch
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#3: Instant Color TV
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#4: The Taco Liberty Bell
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#5: San Serriffe
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#6: Nixon for President
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#7: Alabama Changes the Value of Pi
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#8: The Left-Handed Whopper
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#9: Hotheaded Naked Ice Borers
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#10: Planetary Alignment Decreases Gravity
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Find more information at the Museum of Hoaxes website.