I've always been kinda curious how things might have played out if native American tribes were left to their own devices and had eventually united to form nation-states across the country. What kind of butterfly effect that might have had on other historical events, how they would've dealt with industrialization, etc.
For the more direct approach/sure thing, yeah I'd kill baby Hitler lol. But one does have to wonder if that would prevent World War 2, or simply delay the start of it and change the reasons for which it was fought.
Would they have formed nation states? Without modern communications (and bearing in mind they did not even have horses or a real equivalent) and farming methods (again no horses to drag ploughs around, few crops really worth considering either and geography to grow them dubious at that with the whole north-south thing) you do find a natural limit on the size of empires/countries, populations and the like. There is also the geography question of is it plausible for some kind of industrialised society to have formed even if that is handwaved in some way? Following discovery of it (pretty much inevitable) then you are never going to stop "ooh free land" (or "ooh free converts") and the mass kill off by disease short of dropping modern technology on it as far as medicine, vaccinations and weaponry, which then means probably them expanding out and taking over the rest of the world. That or go really far back and do basic bluewater navigation (open sea navigation and seafaring is hard, most things throughout history splashed around in nice costal waters rarely losing sight of land, also why those which bordered on the north sea tend to make the best seafarers) enough for trade (and horses/wheat/sheep/...) for say Carthage or maybe one of the west African states and that is several thousand years difference at this point. That or going back even further and hoping whatever horse analogue was both useful as a farm animal and not wiped out by the ancestors of the nowadays American Indians and its worth realised which is probably going to be 10000 years difference in history (not to mention probably still no contact with the rest of the world so likely disease issues, maybe with some reverse exchange).
Also yeah I would +1 a Hitler was not really one of those great men of history that did have actions pivot solely around them and would not have come to pass. Some would say the conditions of surrender for world war 1 pretty much ensured world war 2 -- still would have seen hyperinflation, political polarisation, secret army building (it was happening way before Nazis came to power, not to mention the Treaty of Versailles was an awful way of neutering an army), hyper nationalism and whatever else. Sykes-Picot Agreement also ensures the middle east is a hole to this day, granted that did not need much convincing.
Even if it was the case I would have the Hitler vs Stalin question if I am purely about preventing evils of the 20th century (though do I do Stalin or go back further and make it so the white army in Russia wins), and just in Europe (plenty of arseholes in Africa and Asia to consider in this, several others in Europe too -- Franco, Mussolini). 20th century arseholes are also in some ways not even with the high score there -- do I go for Genghis Khan (if you want to talk about ripples then... world changing beyond most others here), Napoleon, king of Belgium (see Belgian congo), Chinese emperors, Chinese communists (not that what was there otherwise was too much better), Japanese warlords, Mughals in India, Shaka Zulu (ripple effects of him are crazy), Bantu expansion (even more ripple effects), Spanish inquisition, ascendancy of the Habsburg dynasty, Attaturk, Muhammed (and he was something of an anomaly as these things go, though it was also considerable work on the part of those after him that turned him from random desert cult leader into something that lasted)... some of those are quite questionable if doing the "unquestionably bad" thing as well.
World war 1 might be easier to stop as that was arguably a series of very unfortunate events. However we then have the concert of Europe thing to consider (world war 1 as it stands was a series of unfortunate events but modern warfare, arguably first seen in a little backwater called the US during its civil war, was a possibility) as it was a well known risk long before then, not to mention Austria-Hungary was pretty much cooked at this point and that was never going to end pretty. The butterflies of that are also considerable and may not make for something those today might like; world war 2 might have brought about the end of empires but 1 put a bullet in the knee of them and changed society quite radically (to say nothing of all those war dead did rather stop the Malthusian Trap problem for a few more decades).
What would stopping world war 2 also do for me? European empires would continue, Americans would probably be isolated (rarely the path to the land of milk and honey if you like modern freedoms and fun there), I don't know what Russia/USSR would have done here and things might have instead not been two fronts and invaded in winter.
For my money. Would probably find a point in history wherein scientific advancement was held back and sort that, and there are a fair few. Depending upon what goes then humans could be centuries ahead of what we have today (though there are some that might argue that is required those centuries to advance the human animal to the point of taking it).
Not sure where and when though, and how to seed it such that further random chance or elites holding onto their power would not snuff it out as they get displaced by said advancements*.
*"You must allow my poor hauliers to earn their bread" seemingly attributed to Roman Emperor Vespasian who died in about 89CE in response to automation of some things there, the concept of technological unemployment going back further still.