Here’s a fun thought experiment. Let’s say you have a time machine. You are given the goal of traveling back in time, with the goal of making the current day as peaceful, prosperous, and democratic as possible. Basically, to have as much of the world possible be 1st world. Here are the rules. You get to take 9 other people. You have two years to prepare, learn the local language and culture, etc. You can choose one land vehicle, a bus, a tank, whatever you choose, fully fueled and packed with any items you choose. You can choose things that will help you prove you are from the future, weapons, anything you think will be useful. If you need medications, those will magically appear, but are only for personal use for you or the other 9 people you bring. You cannot go back to the days when people were still hunter gatherers, you have to go to a civilization, no further back than the first cities in the Middle East. The end date is today, August 29th 2015.
What would be the best choice? The early 20th century to try to make Russia and possibly China develop into first world nations along with the current West? Ghenghis Khan’s time to help him conquer the whole world? The Roman Empire under Trajan? The 18th century British Empire? The Han Empire in ancient China? Please discuss 
Making as much as the globe “First World” developed as possible?
I’d go back to help Genghis Khan, and do it at sword* point. Probably the most efficient way of doing it, and maintaining it, in an orderly manner.
The probably inevitable dips in the population with this method, while tragic, will really only make the job easier. A smaller number of people, and almost by definition the less-defiant ones at that, will be simpler to dominate and work on.
*Well, Sword/gun/rocket/atom point, really.
Rome. Help them make the leap to Franklin stones, bifocals and the beginnings of electricity. Jump start an agricultural revolution with horse collars and such to free up labor and give them the steam engine. Enough of that throughout the Iberian peninsula would help. Jump them from the first century AD to about the 15th or so and see where we end up with a millenium and a half headstart.
You know, less darkness fall or something.
I don’t think there’s any good way to make things much better. War and disease are the biggest problems we face, and war is still happening today, as is infectious disease. If anything, I’d introduce smallpox, bubonic plague, and cholera to the Americas around 500 AD or so, and keep re-introducing it until the natives had built up a good immunity. That would at least slow down the Conquest.