My favorite hoax (April Fool or otherwise) was:

• the War of the Worlds radio broadcast
• Sawing Off of Manhattan Island
• Society for Indecency to Naked Animals
• Uri Geller
• Dihydrogen Monoxide
• Crop circles
• Yellowcake forgery
• Piltdown Man
• Parrot prank
• the Great Spaghetti harvest
• other (but you have to post a note specifying your favorite)

Revised: Parrot prank added thanks to moritz