Kingpin, joker vertex virus.

Keep in mind, Venjix/Evox actually took over an alternate Earth and killed almost all of humanity. And he made another attempt to do so about 9 or 10 years later and nearly succeeded again there.

I will have you know a few things about Venjix Virus/Evox. Venjix was forcefully created by Professor K during her time in a cruel organization. Venjix went self aware and conquered an alternative Earth and nearly killed the entire human race. Venjix created an Skynet like organization and the most dangerous and powerful machines that the Power Rangers had ever fought. He lived in a computer whenever Venjix’s body was destroyed, he went back to his computer. After he was destroyed for the last time, he lived in the RPM Red Ranger’s morpher until he will be revived again one day. He was revived by an curious boy and that very same boy who will be an Power Ranger which will be his permanent downfall once and for all. Venjix is one of the most powerful, heartless, unforgiving, cunning, vicious, darkest, and cruel villains that the Power Rangers had ever fought in their franchise. Joker, on the other hand, his feats were not that close to Venjix’s. Joker wants to make Batman’s life a living hell. Joker has killed thousands of people including Jason Todd the second Robin while Venjix almost killed the ENTIRE human population in an alternative Earth. Joker’s intelligence is not that close to Venjix’s. Joker is an human, so he can die the same way as a human: injuries, aging, and disease. Venjix is an virus, he is nearly immortal. The only way to kill him once and for all is human DNA.

Most vicious? The Joker, no contest.

Most dangerous? This is a hard one. Since we've not seen all that much of MCU Kang yet, I'll say it's a tie between Lucyfer and Venjix. On one hand, Lucyfer IS literally the Devil, but on the other Venjix successfully wiped out an entire alternative Earth worth of people and organic life, save for one city.