"60 million" is far too high - most serious scholars suggest it's closer to 2 (as in probably directly responsible for - tertiary deaths aren't counted as there wasn't an official order). Folks who like to count other stuff put it at 20 - which is probably still far too high given the Soviet population at the time and the fact that 20+ million were killed in WW2. I mean any million of your own people being killed off is too many I would suggest lol. But the old Cold War propaganda of Robert Conquest and the CIA's guesstimates live on and will probably do so for some time.
Note I do not espouse the following views myself, but if you really wanted to troll and go all in, you could. My degree is in Russian History after all.
As an example of easy counter arguments lol.
Leon Trotsky was a traitor to the Soviet Union who deserved death and actively sought to destroy the central government which was lawful. The Bloc of Rights and Trotskyites was *real* though much derided as the subject of show trials - Trotsky *did* actively seek to form a political group within the Soviet Union made up of members of the Bolshevik party which would actively share information so as to do damaged to the Soviet Goverment. Stalin accused them of terrorism and their confessions to as much are questionable at best. As such he was a mortal threat to the government at a time when it was under great threat from abroad. (Justify every misdeed by claiming the USSR was under great pressure and threat from abroad and so was compelled to do strange things).
- Execution of Polish Officials: This certainly happened and the objective was to secretly eliminate people who would be threats to Soviet Governance in former Eastern Poland. Eastern Poland was mostly Ukrainian and Belorussian, but it had been Polish policy in the inter-war years to settle Polish people in these regions to try to ensure they didn't just spontaneously rejoin the USSR. Part of the reason treaty negotiations between the French, British, and Soviets didn't result in a tri-partite military alliance is because Poland outright refused to allow Red Army soldiers into Poland for fear that the Ukrainian and Belorussians living in their Eastern portions would leave Poland in the aftermath. For example Nikita Khrushchev's parents were Polish citizens during the inter-war period. (Again, great foreign pressure, dangerous international situation, extraordinary action).
Order No. 227 is *grossly* misrepresented in the Western press. It was more bombast and propaganda than anything else. Furthermore it was primarily only utilized against persons who actually were failing in their duty - which happened in every army in the war (people who sought to desert or refused to fight). The policy was applied *very* unevenly and in an army that would eventually grow to encompass 34 million people over the course of 5 years, the fact that there were only ~1 million sentences by military court for any actions detrimental to the army, shows that only 2% of the Red Army experienced any sort of legal penalty for their actions - which is more or less on par with the rate of desertion and other various military crimes in the other Allied armies. Even the USA executed a soldier who deserted the night before D-Day (and sentenced 48 others to death as well, though those sentences were eventually commuted). Again threat and international pressure, mortal threat especially etc.
If your intention is to go into the gathering and argue that indeed, Stalin was the greatest Russian leader - I could give you quite a bit of reading material because it does exist. I don't necessarily agree with or even like some of it, but it's there and it's published so you could use it.
- As far as who I think the greatest Russian leader was: You could make *very* strong arguments for the usual suspects: Aleksander Nevsky, Ivan the Terrible, Peter the Great, Czarina Catherine the Great, or even Lenin (though he only ruled for a short very chaotic time). Part of the reason one could argue for Stalin is that he virtually defined, in international politics, a very large portion of the 20th century. Some historians even call the 20th century "stalin's century" because he really was inordinately important and involved in international politics on a global scale.