[dkimball]

My criterion is pre-1945 body count for the governments in question, including non-military actions (like the Holocaust of the Jews, Gypsies, and Slavs; and the Holodomor of the Ukranians) as well as military. It doesn't matter whether or not you count Operation Keelhaul, the campaign against "rootless internationalists," and the like: Stalin worked more people to death in the GULAG network (and starved to death in the special settlements) than Hitler gassed (and worked to death: the Nazi concentration camps were modeled on their Soviet counterparts) in the Holocaust.

The Communists targeted class enemies, while the Nazis targeted racial ones, but this was a less important distinction than you think: if you were a kulak and became poor, you were now a poor kulak, not a proletarian... and in practice, Lenin and especially Stalin managed to off an awfully large number of race enemies anyways.

Edit: For the "worst _criminals_" part, as opposed to "worst faction": look at the crimes that the Nazis committed, that the Japanese committed, and that the Western Allies committed; Stalin committed crimes of the same categories, including crimes which one of these three groups did commit and the other two did not. The Winter War was Stalin's Manchuria; Berlin was his Nanking; his artillery bombardments of cities were his strategic bombing; the Molotov-Ribbentrop Pact was his Molotov-Ribbentrop Pact; his extermination of the Ukranians and the kulaks was his Holocaust (remember: a kulak was a class enemy, but a kulak who became poor was now a poor kulak, not a proletarian); and he committed the Katyn Forest massacre, which was on a larger scale than any comparable massacre done by anyone else.

[DenisM]

The worst criminals in that war were the Communists

Stalin's purges predate 1941 when SU entered the war. So they aren't part of the same war, unless you also want to add the extermination of native americans to the list of WW2 crimes.

And what was wrong with the winter war? A plain old war of agression, sure, but I don't think there were any war crimes?

[dkimball]

The Winter War, like Manchuria, was a "crime against peace" by the standard of Nuremberg; that establishes that Stalin, like both Hitler and Hirohito, had launched a small aggressive war prior to WWII.

The purges were not part of the war any more than the Holocaust was part of the war (pointed out at Adolf Eichmann's trial: the continued prosecution of the Holocaust actively impeded the Nazi war effort, especially by clogging up the railroad network); but I wanted to make it clear that Stalin as well as Hitler exterminated large numbers of his subjects during his "reign."

[DenisM]

Well, "the crime against peace" is a single-serving "let's stick something on them" charge. US engaged in multiple aggressive conquests after WW2 without anyone even mentioning the word "crime" in a serious context. So aggressive conquest is a common practice, whereas civilian massacre is universally frowned upon - a big difference in my mind.

I can see how to tie holocaust to WW2 - the regime which perpetrated the former has then also started the latter. Stalin didn't launch the WW2, but then you can argue he did set it in motion with the Ribbentrop-Molotov pact making both the purges and the WW2 part of his empire-building. Good point, I agree.