The lightning bottle. Ion Engine - Couldn't have the TIE without 'em. Movable type just wasn't that valuable in the East. I have to agree with A.Coward. I have always said refrigeration to be the best invention ever.
Vaccines come a very close second.
Yes Gutenberg did concieve the idea of printing with movable type. However many people are taught he was the first, because many are taught history in the western point of view. He carved characters onto clay and slowly heated them up, however it proved impractacle because the Chinese language contained 40,000 characters and the clay type shrunk irregularly. Koreans then developed the world's first metallic movable type a few centuries later. Some texts say Gutenberg invented metal movable type, some texts say Gutenberg invented movable type, and some texts say Gutenberg invented the printing press.