Pre-1900, I don't think you're likely to see too much revolution. If Russia messes up (like in 1905), maybe one of the leftist groups could claw their way to the top of the resulting power struggle. Speaking of 19th century Russian leftism, something in me would love to see Sergey Nechayev in a prominent role for a post-revolutionary Russia (avert his death in prison to start). But most realistically, a much more extreme capitalism and conservative/liberal groups that refuse reform for reason or another would be able to provoke
something, although they'd be most likely to produce results post 1900. That's pretty hard, considering the circumstances in the late 19th century. If they do so, then that leaves Britain and France as the most likely centers of revolution, as in, turn the Paris Commune national, or produce an event like it soon enough after with the utter failure of the Third Republic or any alternative. Britain might do so too if they mess up enough, but France seems more likely for some reason. Either way, their socialism will be different than what we saw in Russia in 1917-1991 and leave a far different mark on workers' movements than what the Soviet Union (for better or worse) did.
Other less likely nations are Germany and Italy. These countries have nationalism and revanchism as an outlet, as well as emigration--look at all the left-wing Italian and German immigrants. Scandinavia seems more likely to reform as a social democracy (as OTL), although once again, it has emigration as a good outlet--many Scandinavians were loyal Socialist Party members in the US (Finns most especially, but in that time period they were of course Russian technically) and helped the Great Plains be an important part of Socialist activity in the US.
I mention the US, but the US will need Progressivism and other movements fail on the ground combined with, as in Europe, capitalists acting worse and worse. That's really, really hard, considering how awful people suffered under their system until they got some manner of reform--seriously, talk to Americans who vote solid Republican, and many of them will admit the system was flawed in that era. So the key is to keep it flawed and let the tension associate in the Socialist Party which will drift to the left. But that will take a while--the US still has the vast frontier, which was still being settled, and that helps as an outlet. I'd really say that's a factor until the early 1930s with the Dust Bowl, which helped immensely to kill the idea that no matter who you are (an immigrant, whoever), you can get your own land and be your own master on the "frontier"--there were other events in the early 20th century. But in the 19th century, that idea was alive.
I think Britain is likely to dominate something akin to the Social Democracy, or the Soviet Union period of NEP. Spain - radical Marxism, anarcho-communism. France - non-Marxist socialism, a far-left currents (eg Trotskyism).
Could Spain really go communist without the help of an external influence, like the Soviets? Sure, the Second Republic was pretty radical, but not a communist state. And the Spanish far-left was split between communists and anarchists, with anarchists dominating the earlier you go.
British communism is very interesting. It'll be more democratic, without a doubt. What that will lead to I'm not sure. If Russian communism was some mirror of Tsarism, would British communism be a mirror of the British Empire before it? Likewise with French communism, which will have a huge dose of French Revolutionary tradition to fall back upon (for instance, the French Revolution calendar will come back, as it did in the Paris commune, guaranteed). Granted, all non-monarchist French idolised the French Revolution to some degree or another, but French socialists will find quite a bit in it. There's also the non-Marxian socialist tradition in France to consider--Trotskyism I doubt, since that still owes a lot to Leninism which in turn is too Russian to originate independently in France.