Man this is one hell of a subjective question.

The worst currently living dictator would most probably be either Than Shwe (senior general in Burma's military junta) or Robert Mugabe ("president" of Zimbabwe).

I hesitate to put Kim Jong Un on the list only because right now it's looking more and more like he's being propped up as a figurehead while the real nastiness and string pulling is being handled behind the scenes by the top generals of the North Korean military. If King Jong Il were still alive I'd put him in the #1 spot for worst dictator, but he's not, and Kim Jong Un looks more like an impotent little leader waddling about for the cameras than any sort of true mastermind.

As for the worst all time dictator? That one's really too difficult to answer, history is littered with some nasty pieces of work going all the way back past the Bronze Age. If you had to go by kill count alone, then Mao Zedong would take the #1 slot. His inept policies and ruthless revolution resulted in the deaths of over 50 million people. However, there are differing levels of criteria that need to be taken into account besides just kill count. You'd also need to take into account crippling economic policies, elimination of basic human rights, forced indoctrination of state propaganda, ethnic cleansing, destructive agricultural policies, destructive food distribution policies, territorial expansion through hostile annexation, and so on. Under that criteria, when you count for scale, you'd probably end up with either Joseph Stalin or Genghis Khan taking home the gold for history's worst dictator.

This submission has been removed because it involves current events. To keep from discussion of politics, we have a 20-year rule here. You may want to try /r/ask_politics or another current-events focused sub.

This submission has been removed because it violates the rule on poll-type questions. These poll-type questions do not lend themselves to answers with a firm foundation in sources and research, and the resulting threads usually turn into monsters with enormous speculation and little focussed discussion. “Most”, “least”, "best" and "worst" questions usually lead to vague, subjective, and speculative answers.

For questions of these types, we ask that you redirect them to more appropriate subreddits, such as /r/history or /r/askhistory.

Sorry, we don't allow throughout history questions. These tend to produce threads which are collections of trivia, not the in-depth discussions about a particular topic we're looking for. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again. Alternatively, questions of this type can be directed to more appropriate subreddits, such as /r/history or /r/askhistory.

This submission has been removed because it involves current events. To keep from discussion of politics, we have a 20-year rule here. You may want to try /r/ask_politics or another current-events focused sub.

This submission has been removed because it violates the rule on poll-type questions. These poll-type questions do not lend themselves to answers with a firm foundation in sources and research, and the resulting threads usually turn into monsters with enormous speculation and little focussed discussion. “Most”, “least”, "best" and "worst" questions usually lead to vague, subjective, and speculative answers.

For questions of these types, we ask that you redirect them to more appropriate subreddits, such as /r/history or /r/askhistory.

Sorry, we don't allow throughout history questions. These tend to produce threads which are collections of trivia, not the in-depth discussions about a particular topic we're looking for. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again. Alternatively, questions of this type can be directed to more appropriate subreddits, such as /r/history or /r/askhistory.