I think these are the funniest April Fool's prank's that I have ever read.
1) Burger King, an American fast-food chain, published a full-page advertisement in USA Today in 1998 announcing the introduction of the "Left-Handed Whopper," specially designed for the 32 million left-handed Americans. According to the advertisement, the new burger included the same ingredients as the original, but the condiments were rotated 180 degrees. The chain said it received thousands of requests for the new burger, as well as orders for the original "right-handed" version.
2) In 1996 the Taco Bell Corporation announced that it had bought the Liberty Bell from the federal government and was renaming it the Taco Liberty Bell. Hundreds of outraged citizens called up the National Historic Park in Philadelphia where the bell is housed to express their anger. Their nerves were only calmed when Taco Bell revealed that it was all a practical joke a few hours later. The best line inspired by the affair came when White House press secretary Mike McCurry was asked about the sale, and he responded that the Lincoln Memorial had also been sold, though to a different corporation, and would now be known as the Ford Lincoln Mercury Memorial.