Assuming that the left gets its shit together before Homo sapiens goes extinct.
My vote goes to either the USA or France. Especially because the Paris Commune already happened. “Smeagol did it once...he can do it again!” Paris seems to have been packed with rioters 24/7 for several years. Not so different maybe from the American Midwest. The ruling class is investing heavily in police in both countries. It’s afraid!
I know this seems partly delusional of me and that there is far more hope in the Global South unifying and then strangling the imperial core from the outside. But still, gun to your head, pick an imperial core country and explain how and why it will fall first. Marx, after all, predicted that socialism would come first to those places that had progressed along capitalist development the most. (Yes, you are invited to explain how wrong I am about this.) The USA falling to socialism would probably cause the rest of the planet to fall the same way.
edit: my choices seem embarrassing now, thank you for enlightening me comrades.
I think at the time Marx thought the proletariat had the most revolutionary potential but in the first world where capitalists have stratified the working class further it's only those who are the furtherest exploited that have any real motivation to change things because they lack the security of the labor aristocrats and petty bourgeois. This is also the MLM approach to building revolution in the first world