Hard question as well...
I would be tempted to go back and kill Jesus or Hitler before they grow up and cause millions of deaths, but I would probably pass as I cannot guarantee that it would make the world better place. Maybe there would be even worse people to replace them. So I would probably go to around 1900 to chill out with Nikola Tesla, tell him about Internet and shit. He would love that.
If you would be about to be sent back 600 years back and stay there, what 3 things would you bring with you?