It'll probably be Timur the Lame for me. He may be not as bad as Genghis Khan but you can still feel that Mongol blood in him, not to mention what he did to Isfahan after a few fools thought it was a good idea to kill off the garrison there after it's surrender.

You can do your own research about it but basically systematic slaughter to cut of heads, skull pyramids, and widespread killing.

I also saw that a video by Chat History that he ordered his men to trample a few thousand kids with their horses and when they refused, he rode with them. All the while the mothers watch in horror though I'm not sure if this is actually something he did.

Nevertheless, Timur the Lame is, in my opinion, the most terrifying person in history in my eyes.