If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
What Earth-like world would maximize the amount of good a single person could achieve?
I don't actually believe the following answer, but as we're talking about fiction, and some people might actually think it would work, why not.
A world in which a catastrophe has reduced the population to a group of a few breeding pairs, or just one -- plus yourself, who have managed to survive the catastrophe and preserve your access to the technology of the vanquished civilisation. You use your overwhelming power to take charge of the remaining tribe and direct their development, confident that you can do better than the blind chances of history. You crack some sort of good-enough immortality for yourself, so that you can perpetuate your absolute rule indefinitely, and make humans live they way the clearly ought to. Since you are, for practical purposes, omnipotent, omniscient, and omnibenevolent, failure would be a contradiction in terms.
This one sounds like a lot of work to do - I don't think I mentioned that you have the power to do this as often as you like (and have time for!) and this would tie you to just a single world forever instead of doing as much good as possible across many worlds. I'd also like to not assume any access to omnipotence, immortality, or other such things: you can specify a world to travel to by describing and then travel to it. But once you're there you're just like anyone else, save for the knowledge that this world matched your description and any knowledge yo...