Personally, I'm too much of a trouble maker. Knowing how warfare works, and the different strategies (successful and otherwise). I also know plenty about disease, technology, and where certain valuable minerals are located. I would go back to medieval Europe, take it over, and from there take over the world. The glorious empire would rise. I also know a decent amount of what governments work and how corruption sets in (no government is without corruption). The technology I would bring (similar to the aquaducts of Rome) would be hailed by everyone. I would also know how to get rid of that whole black plague thing.