I chose Germany.
I assume that Britain wins the independence war in America, and tries to keep those colonies from becoming too strong. That also means, other powers like France, Russia and Spain keep larger interests in the area - and therefore more of the ressources there. The Spanish states may become independent similar to OTL, but reach far more to the north. The French states will be gobbled up by Britain, similar to OTL. Russia extends Alaska slightly, as there is no such rush to the west from the eastern US.
WWI happens similar to OTL - while there is no US to help the Allies economically and later with some participation, GB is a good deal stronger due to rust belt ressources. The war might even be over in 1917 already. Still, as IOTL, the war costs Britain too much to keep the Empire together - The US emerges around 1920, much smaller and weaker than IOTL, and forced to build up an industry which doesn't exist to such a degree at that time, due to Britain monopolising manufacturing to a large degree. Similar to Ireland, it'll stay completely out of the war.
Germany thinks in terms of revenge similar to OTL, and is even more able than IOTL to break any treaties and conventions, as no serious US threat looms.
In WWII, it was mainly US help to Russia and Britain which kept those alive. This help is drastically reduced, and much more susceptible to naval disruptions, as the industries in Britain are crumbling (lost North American ressources and markets) but still taking up much of the ressources of the former colonies, while the industries in the US are still not strong enough.
There is no big change in economic power in Russia - even Stalin is still in power (for simplicity's sake).
Market economy is even less popular than IOTL, due to the lack of a well known US example, so that Britain has reduced growth in the 1920s and 30s, while Germany's aproach delivers similar results to OTL. Also, as some of the increased military spending increases already start in the late phases of the Weimar Republic, due to no US ensuring peace and the Versailles Treaty, Germany starts into fascism from a slightly "better" position - especially in terms of military technology. Military planners during the Weymar Republic were probably slightly better than Hitler's rather amateur-like approach to directing ressources - there would be more and better subs, more and better torpedo boats, more and better torpedo planes, better torpedoes, and so on - Germany would be prepared for cutting the lifelines of the British Empire. And it would also be stronger in other areas.
The result is that the Britons don't have a choice but to agree to peace with Germany shortly after the fall of France.
Germany would still fight the war in the east, even from a better position. Despite some advantages (like more and slightly better planes available), it would still loose the one or other battles eventually - thus stopping the fight for "Lebensraum" somewhere between 1943 and 1945, pretty much victoriously. Continental Europe will be pretty much German territory, though Spain, Italy, and other places will officially stay independent.
The US will as well be the first country to develop the nuclear bomb, thanks to Einstein, among others. But as the war in Europe is over by then, Germany will not be targeted. Also, the US won't have as large or advanced bombers available. Instead, the fascist victory will influence US opinion a lot - communist scare will include hostility towards Jews. Separation, eugenics, and similar stuff will reach new "heights" (world wide).
Germany will easily be able to keep Israel from forming, by supporting their enemies (similar to Spain), thus decimating the Jewish population even more than IOTL.
The growth of the German industry will be less than what happened after WWII, also thanks to nationalised new industries - but as Germany starts from a better position and with much more ressouces available, it'll still be the industrial, technological, and military leader at the end of the 20th century.
The Commonwealth will be much stronger than IOTL, as the Commonwealth countries will be under far more pressure to compete than IOTL under a US lead cold war "umbrella". The US will also be a member. The Commonwealth will be something like the EU, but more militaristic. It's members seen as one will be the second most powerful block.
China will not be allowed to become communist - especially as Russia is in no position to support communism there to a large degree. Instead, maybe Manchukuo and Tibet stay/become independent after WWII. Inner Mongolia will be the area were the communist Chinese will retreat to and will unite with Mongolia eventually. Capitalist rump China will grow much faster much sooner (not quite as much as Taiwan did IOTL, but still impressive).
Capitalism and democracy would be completely discredited in Germany - there would be no movements at all in this direction, and there would be quite a few countries following that example. As quite a bit of the German success was built upon the achievements of the Weimar Republic, Germany would loose it's leadership at about today. According reform attempts would lead to another wave of self destruction due to the fascists fighting the reformers successfully while the number of people realising that reforms are needed reaches higher and higher levels. More and more wars in foreign countries are lost by Germany. The US and China emerge as the new leaders.