It depends on what you consider to be success. Marxist-Leninist communists uphold that there are five currently existing socialist states, China, Cuba, Laos, Vietnam, and the DPRK (North Korea), so if you're judging by "not having fallen", then those five would be successful.

If you want to look at things such as standard of living, though, which I feel would be a fair one to look at, they were all very successful. Across the board, the difference in standard of living in socialist states before and after Communist parties came to power is practically night and day; regardless of one's stance on their governing philosophy, the tremendous advances made across the board in all socialist states, past and present, in areas such as life expectancy, infant mortality, universal education and healthcare, and constitutionally enshrined rights to housing, food, and employment, cannot and should not be ignored.

As an example, in China between 1949 and 1975, life expectancy doubled, from 32 to 65 years. In 1949, some 15% of the population was literate; in 1975, it fluctuated from region to region, but overall it was roughly 80-90%. In the early 1970s, infant mortality in Shanghai was lower than in New York City.

As another example, in one of the lesser known socialist states - Burkina Faso - in the four years that the Marxist leader Thomas Sankara was in charge before his assassination, Burkina Faso, today a third world country, was able to achieve food self-sufficiency (!) by more than doubling farmland productivity, vaccinate more than two million in the span of a single week via government-funded programs, lay 700 kilometers of rail to better connect the country, and even combat desertification in the northern reaches of the country bordering the Sahara.

And conversely, by looking at the successor states to the thus far fallen socialist governments, we can also infer conclusions. In the former Soviet bloc, for example, rates of drug addiction, alcoholism, domestic violence, infant mortality, and suicide have all been steadily on the rise since the fall of the USSR, though it was particularly horrifying in the immediate aftermath, throughout the 1990s. Life expectancy in Russia today is lower than it was under Stalin in the late 1940s. There's a reason that more than half of Russians want the Soviet Union back.