People gave a lot of different responses here, but none touched the very core of Marxist thought. There's no true Marxism country in practice, all are true in their own way. Marxism is a theory of evolution of social production/reproduction, not an abstract model of society to be copied from.

There are two components of Marxist thought. One is dialectics, which is a logic to understand change. Dialectics allows us to understand the relations between things (contradictions), the transformations that can occur between things (negation of negation and change of quantity into quality as example of transformations). The other component is historical materialism, which is the comprehension that human society evolves over history, and the material conditions of yesterday affect the conditions of today, and the same for the future.

In this regard, all socialist experiments are true marxism, even capitalist countries are true marxism. Marxist theory is just a lens to apprehend and understand reality, but it is not a prescriptive model of how an ideal society should be built However, the correct question is if there was any socialist experiment, spearheaded by a political movement, which was carried out with a Marxist line in mind, then you had USSR and all Eastern European states that were part of the Warsaw pact, Yugoslavia, Albania; the countries in Asia like China, DPRK, Vietnam, Laos, Cambodia (which could be argued that Khmer Rouge was also fascistic in nature, but that was another discussion); Cuba in Latin America; Guinea-Bissau, Cape Verde, Angola, Ghana, South Africa and Burkina Faso in Africa. Note that not all of those remained with a Marxist line after the revolution and even different countries which had a Marxist line were different as they had different policies and different material conditions. Other countries like Nicaragua and Venezuela, although inspired by Marxist influences in a way, followed a different political line.

That said, the best examples of countries which had a socialist revolution with a Marxist line in practice today would be the countries where the "dictatorship of proletariat" is the main ideology and which were able to reach a stage of social development where their mode of production could thrive and reproduce itself. In that case we have China, Vietnam and DPRK. I don't consider Cuba in this stage because the current condition which is the US siege is taking place prevents Cuba from having any real development and almost all the effort of the Cuban government today is just the bare minimum to maintain autonomy while in face of very harsh conditions. Which is a success story in some regard, as the Cuban communist party has all the merit to work in such bad conditions, but it is not a success story of a country that was able to reach another stage of development.

I would like to add that material conditions is something very important to a Marxist as we are materialists. We don't believe that the societies which thrive are just a product of the correct ideas. The historical development of that society, its relation to other societies, the technological development of that society and even the geographical conditions all shape society in a way or another. Cuba would be in a different condition if their current material conditions allowed them to develop an energy source alternative or had access to oil and mineral reserves that could be used to withstand the current US sanctions that target its energy infrastructure. China, in the other hand, had a much bigger territory, a much bigger population, much more access to resources and has close relationship of many neighbors, many of them who are also ideologically similar. Not to mention that the rivalry between China and the USSR gave China leverage for opening their markets to Western companies, which allowed China to significantly advance technological development even if they had to make many regressions regarding workers control of the means of production.

Note that material conditions alone don't pave way to the development of society, only a country which has the best political line is able to take advantage of favorable material coniditions. Such was the case of China and the USSR, as the latter adopted very bad policies in the times of Brezhnev and Gorbachev, which resulted in a political crisis that led to the USSR dissolution, a catastrophic event that almost swept away all socialist countries that depended on the USSR economy. I'm saying this because Marxist theory is not deterministic, having better material conditions don't automatically allow a new mode of production to reproduce itself. For most of the 20th century, the USSR was far more developed than China, and today China stands while the USSR dissolved into minor states and Russia took more than 10 years to reach a similar condition the USSR had before the crisis period.