There are probably some on a similar level, but I think he’s the worst…takes a special kind of evil to systemically murder your own citizens. People like the Khans and Huns raided, but in the pre-modern world that was more typical fare.
The likes of Genghis Khan and Tamerlane killed on an industrial scale - and there is a good case that the Mongol sack of Baghdad in 1258 is one of the single most catastrophic events in human history; maybe the biggest loss to our knowledge base since the library in Alexandria burnt down.
Good to see Macias Nguema in this poll btw - anybody who doesn't know about him already could do worse than to read the Wikpedia article on him, which leaves out several bizarre and/or shocking details. And if he and Amin are on this, Bokassa should be too - he had schoolkids slaughtered en masse (shot/clubbed to death) for not buying his personally designed expensive school uniforms FFS.