Who was the worst tyrant of the 20th century?

What makes this such a sad question is that the tyrant that your government wants to keep on the good side of, or get something out of, like oil, is never painted as a tyrant. It's only the ones they don't agree with. Margaret Thatcher was at times verging on being tyrannical in her term as UK Prime Minister. In fact it is partly her tendency to this that caused her to be deposed by her party.