I'll add another vote for the early United States. I think a lot of people on here forget that the US in 1812 was far from the superpower it is today, or even the great power it became in the later 19th century.
Edit: could the USA conquer Canada in 1812? Or could the USA conquer and rule all of Mexico
I'd say no to both, assuming the latter is referring to the Mexican-American war. The only way the US is getting Canada is if the British decide they'd like to divest themselves of
quelques arpents de neige and sell it. OTL showed that the British in 1812 had the power and will to strangle the East Coast economy. New England was already hit pretty hard OTL so I don't see why an ATL could go better for them.
For the second, could the US defeat Mexico's conventional forces? Easily. Could they
maintain rule over a restive population of millions of restive Latino Catholics in a country that is anti-Latino and anti-Catholic? I don't think so. It would be like Prussia attempting to annex all of France in 1871. Sure, the regular army is defeated but good luck getting a population with a well-developed national identity to submit to your rule without an expensive occupation force. Not only is it a huge distance to project power from the American "core" which at this time is still the East Coast, the terrain of Mexico makes it super easy for guerilla movements and secessionist movements to thrive.
Also, I'd add to the list Ethiopia. As per my username I do think Ethiopia is super cool, but I often see it mentioned in countries that could "pull a Meiji" in the 19th century and I just don't think that's realistic given the resources, population, and location of Ethiopia. Ethiopia didn't have the high literacy rate of Japan, nor did it have the practice of Rangaku where the Japanese studied European tech for a long time before Meiji. And while Japan entered the Meiji reformation after a long period of peaceful internal consolidation and population growth, Ethiopia until the 1850s was still wracked with internal conflict, the Zemene Messafint. Ethiopia could definitely turn out better than OTL given some PODs IMO, but the concept it would become an African Japan is not realistic to me, especially when there are many much better candidates.