I think I would probably stop Hitler's rise to power, which would have significantly altered history in the last 75 years. Not sure if the world would ultimately end up better, but I think stopping Hitler's rise to power certainly alters when (or if) WWII happens, the cold war doesn't happen when it does (or at all), heck even Japan probably doesn't attack the US at Pearl Harbor without the war in Europe, etc.
What say you other celticsbloggers?
I thought about this recently because I was organizing my closet and came across Orson Scott Card's book Past*watch, the Redemption of Christopher Columbus, where future people go back in time and alter Columbus' landing (they keep him in the new world and start a colony and enter the idea of Jesus into the natives so when the next set of explorers come they are met with a different type of native).