Nero
Hitler
Stalin
Saddam
Osama Bin Laden
the Iranian President
Vladimir

Mao should be on there, and Pol Pot.
Osama bin Laden isn't a tyrant.
By Vladimir, do you mean Vladimir Putin?

Just a quick point: the word Tyrant does not actually mean a brutal dictator who kills and oppresses. It was a Greek form of government where the city-state was ruled by a popular autocrat, with military command. If he became unpopular, the people would simply overthrow him. Many Greeks felt it to be one of the best forms of government.

Also, for a "Most evil tyrants in History" it's a very 20th century based list.

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... I am surprised at your insolence in writing to me at all. You know, as I know, that I bought this constituency... may God's curse light upon you and may it make your women as open and as free to the excise officers as your wives and daughters have always been to me while I have represented your scoundrel corporation.
I have the honour to be... your obliged humble servant, Anthony Henley
- MPs reply to constituent, mid 1700s

The iranian president is a figure without any concrete political power , so he can´t be a tyrant by definition. The iranian system has more than one political pole and its not the president making the final decisions, but the "guardians council". Also while Irans human rights record is appaling to call it one of the worst in history is extremely exaggerated.
What about including a couple of latin- american dictators ( Somoza, Duvalier), the north korean Kim dynasty ( father and son) and the likes of Mussolini and Franco?
Concerning Saddam the middle east has seen many bloodthirsty despots over many centuries. While he definitely belongs in that category there were others with a greater thirst....

one candidate for your list died today. But his has still quite impressing references ...

Al Jazeera English - Central/S. Asia

He wasn't as bad as many leaders in terms of human rights abuses, etc. Completely bonkers though.

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... I am surprised at your insolence in writing to me at all. You know, as I know, that I bought this constituency... may God's curse light upon you and may it make your women as open and as free to the excise officers as your wives and daughters have always been to me while I have represented your scoundrel corporation.
I have the honour to be... your obliged humble servant, Anthony Henley
- MPs reply to constituent, mid 1700s

I was wondering about Vladimir myself.

Lenin's first name was Valdimir.
Vlad(imir?) Dracul "the Impaler" might be another candidate.

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From the fury of the Northmen, Good Lord deliver us.

I think is Genghis khan or his son: Kublai Khan

Mao ravaged China a lot worse than the Mongols.

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chicken butt

True, but then the Mongols didn't just ravage China, they also attacked Japan, Persia, Russia, Georgia, Poland, Hungary, the central Asian -stan countries...

__________________
... I am surprised at your insolence in writing to me at all. You know, as I know, that I bought this constituency... may God's curse light upon you and may it make your women as open and as free to the excise officers as your wives and daughters have always been to me while I have represented your scoundrel corporation.
I have the honour to be... your obliged humble servant, Anthony Henley
- MPs reply to constituent, mid 1700s

Well, I don't know about all-time, but in terms of today, this Kim Jong-il guy from N. Korea is pretty bad. Did any of you guys see the footage from Oprah when an American female went undercover in North Korea? This guy pretty much demands to be worshipped like a god. Pretty freakin' crazy stuff, man.

I was reading in Parade magazine that it's even a serious offense for his pictures (in your living room) to collect dust or have creases from being folded.

Makes me wonder how anyone can even sit down and talk sense into this guy.