I would bring back an invention from the future.
Something from the future can be of so much more help to humankind than something from the past, in my opinion.
I would bring back something that would enhance our health. For example, an alternate fuel source (and method to make or find more of it) so we can cut pollution. Alternatively, some kind of medical invention to help us live longer and healthier lives, even possibly incorporating nano technology (as Loreth has said).