I spent like 5 minutes contemplating this before answering and I decided I want to answer it a bit differently than my initial plan. At first I was going to try to figure out which human has caused the most suffering and death and call that person the worst tyrant. However I've decided to use the other interpretation of "Worst" as in who sucked at being a tyrant the most.
In that sense, I'd say the worst tyrant might have been Caligula. With all the power he had back then he couldn't even protect himself from the people in his house from stabbing him in the back. What a failure of a tyrant, he spent too much time following those sexual pursuits rather than paying attention to the political situation and emotional states of the people he hung around.