• 1921-1922 (Povolzhye, or Volga famine), 5-10 millions dreath
• 1932-1933 (Holodomor), 3.5 to 7 millions death in Ukraine alone
• 1930-1933 (Asharshylyk), 1.5 million deaths (seem small, but that was 40% of then Kazakhstan population)
• 1932-1933 (at the same time than the Holodomor, but in Russia) : 1 to 2 millions deaths
• 1946-1947: 1 to 1.5 millions deaths

And that's only those who were big enough to be impossible to hide completely.

All of them have something in common: the central government minimised them, and tried to hide them. Some weren't even acknowledged until after the USSR fall. All of them are a combination of bad luck (war, drought) combined with hasty decisions which made what could have been a hard year a generational disaster.