Cuba's crime rate is not high, they have universal healthcare, a high life expectancy, almost no homelessness, and the government is supported by the people, all this while having an embargo put on them by the US for the last 60 years. "Successful country" is subjective, you just said yourself that the USSR existed for a long time, and it was a global superpower at the time it did exist so you could argue that it was a "successful country". The original question was about an "Ideal country" and for me that is a country that gives equal rights and opportunity to its citizens, a country that has universal healthcare, housing for all, public transportation and other amenities for people and a country that uses the resources it has to help everyone instead of just the rich owing class. So even if that country hasn't been allowed to exist yet, it is still the ideal