The comparison to Churchill (or FDR) is a good starting point. When libs talk about these “heroes,” their positive accomplishments are what’s important, and any genocide, segregation, forced displacement, etc. that they had a hand in is a mere uncomfortable blemish on a generally good record. They were products of their time working in imperfect systems, they made some progress in some places, their replacement would have been worse, etc.

I say that’s about how I view Stalin – 70% good, 30% bad – except his state was generally pursuing worthy aims instead of Churchill or FDR, who were presiding over and expanding exploitative empires.

There are usually two directions the conversation goes from here:

1. Specific, objectionable Soviet practices. Internal deportations of ethic groups, for example. Getting into the weeds is almost always useless, because unless you and the person you’re speaking with sit down and read some books together (and unless they accept that those books are accurate), you’re going to get lost in disagreements about what actually happened. Best to stick with pointing out that your Churchills or FDRs did the same stuff (Japanese internment, for example), and that at worst they are viewed as flawed, not the worst dictator ever.
2. “You’re doing some kind of fallacy by comparing the USSR to other countries.” Here, I usually point out that comparing state actions is a foundational part of what passes for international law. We’re not judging states against the People’s Republic of Heaven, we’re asking if one system is better than another. Comparisons to other countries are also inherent in any discussion of international relations, which definitionally involve other countries.