Communism

In my experience, you gotta acknowledge the errors first - the purges spiraled far out of control, forced resettlement and sedentization of nomadic nations was unjustifiable, power was too centralized in the party at certain periods, etc etc. If you can't do that, you don't just look like a loon, but you're not learning the full lessons our revolutionary predecessors fought to uncover for us. That leads right into the next step: our goal is to build on successes while avoiding the errors of our ideological precursors.

You have to convince the person in front of you that any evaluation must account for the full complexity of the situation. Refute idealistic notions of an absolute essence that claims every course of action undertaken in the USSR (or whatever project you're discussing) inseperable. You'll hear people say "the USSR was evil because it killed ten million Ukrainians" or whatever and use that to discount elements of the socialist project that can be separated out. I like to isolate in particular the value of the planned economy in producing positive gains for the workers and peasants. What about that necessitates the violent excesses of the USSR? Yes, the USSR did things wrong, but it quite clearly did many things right, and those are the parts we want to draw on productively. Demonstrate the poverty reduction, universal housing and healthcare, scientific and ecological advancement, women's rights, etc. Challenge them to think how we could recreate the systems that produced those successes without repeating the failures.

From there, you could take a few different approaches based on the person's general vibe.

With a lib, the focus is on demonstrating just how bad the US is in contrast. You could talk about imperialism, but they probably won't understand it. Instead I'd start by discussing economic degradation in the US and contrast it with China. China perfectly demonstrates a more 'moderate' socialism than the USSR, and the take on it among libs is improving (Redditors don't count as human in this discussion). Why could we not emulate the 'birdcage economy' in the US? What actually prevents us from nationalizing ~50% of the economy, focused on the commanding heights, and what could we achieve if we were able to do that?

With an anarchist, I talk about Venezuela. Discuss how Venezuela draws on the successes and failures of 20th century state socialism, applies those lessons to their context, and keeps the revolution dynamic. First they tried ML-style SOEs, but that hit its limits in expanding socialism throughout society even if it secured large material gains. Then they expanded to a cooperative Yugoslavia style system, but it stalled out by recreating a lot of the basic contradictions of capitalism. Both of those elements were retained, but the real advancement that anarchists should vibe with is the centrality of the communal project. After the intensification of the sanctions regime under Trump 1, the Venezuelan state responded by prioritizing the construction of true communes: self-governing units built from the bottom up, not imposed or structured by the state. The state dedicates substantial economic and coordinating resources to help the communes develop, build a communal network, and progress towards a 'communal state' that can fully replace the existing state and advance towards communism. Under an ML state that applies these lessons, anarchists have the choice of either struggling against the revolutionary state or struggling against private property with state support. They could dedicate their lives to the construction of fully horizontal democratic communes with state funding. Would that not be a massive improvement over what they are able to achieve under capitalism?