I find this sort of thing fascinating, because it is both speculative and useless, qualities I highly value in projects on which I expend a lot of thought. Some of the comments are funny and clever, some are pretty moronic, but all certainly set you to thinking. In my case, I very seriously doubt I could accomplish the sorts of things most frequently suggested, given that I lack most practical skills and don't know anything about engineering or applied science. I might be able to make it as a military leader, because I know a lot about strategy and tactics and physically, I would be at least a foot taller than most anyone else I met, but that would still be a ticket to an early grave. I do know a lot about European history, but most of what I could tell people would involve predicting horrible disasters, so eventually I would probably just be shunned or killed. My knowledge of basic hygiene would certainly be useful, but I'm not sure if I could accumulate power and influence just by telling people to wash their hands and not throw shit out the window.
Probably my best hope would be to amass lots of money (first through wagers, then managing small-scale resources, and then through the introduction and master of modern financial instruments), and then by prematurely birthing latter-day concepts like democracy, trade unions, psychology, sociology and certain aspects of Machiavellian politics. I'd still probably either get assassinated or outstripped by my own creations, but assuming I could manage to avoid getting buboes or what have you, I might end up as a widely reviled but begrudgingly respected elder statesman of sorts. Also I would invent baseball.
What about you?