I got to #1 as Scandanavia as a liberal democracy (partially because Great Britain and France got involved in a series of colonial wars, so France was never able to colonize N.Africa, letting me pick all of that prestige up), then cheated myself to a proletarian dictatorship in the 1890's. Interestingly enough, it seemed to radicalize a lot of pops outside of my country as well (though the fact that I took a large portion of northern Germany and Saint Petersburg would explain it to some degree), but I had HUGE amounts of immigration into my country, primarily of socialists or fascists.
I'll post a thread in VIP about communism, because I admittedly don't know a lot a lot on the history of communism, but it was nearly impossible to play, even though I stayed #1 for the whole game. I needed to have full reforms on all but workhours + subsidized tarrifs to the max (I've never even subsidized tarrifs before!) to keep even the socialist and communist pops's militancy down, and even then I was getting revolts everywhere and it is IMPOSSIBLE to expand my army in any meaningful way because even as a communist state I am making a small surplus.
edit-also America went Fascist, and apparently Scandanavia's communist flag is the Confederate flag.