Soviet Union, Stalin's regime (1924-53): 20 000 000
There are basically two schools of thought when it comes to the number who died at Stalin's hands. There's the "Why doesn't anyone realize that communism is the absolutely worst thing ever to hit the human race, without exception, even worse than both world wars, the slave trade and bubonic plague all put together?" school, and there's the "Come on, stop exaggerating. The truth is horrifying enough without you pulling numbers out of thin air" school. The two schools are generally associated with the right and left wings of the political spectrum, and they often accuse each other of being blinded by prejudice, stubbornly refusing to admit the truth, and maybe even having a hidden agenda. Also, both sides claim that recent access to former Soviet archives has proven that their side is right.
Here are a few illustrative estimates from the Big Numbers school:
Adler, N., Victims of Soviet Terror, 1993 cites these:
Chistyakovoy, V. (Neva, no.10): 20 million killed during the 1930s.
Dyadkin, I.G. (Demograficheskaya statistika neyestestvennoy smertnosti v SSSR 1918-1956 ): 56 to 62 million "unnatural deaths" for the USSR overall, with 34 to 49 million under Stalin.
Gold, John.: 50-60 million.