By the way I just want to say this right now, Deathstroke is no king of supervillains. I mean I like this accepting he's evil direction but come on he doesn't come close to being a metaphorical king. Even Lex when he was in his Apex/Doom War mode he had almost every big named villain working for him.
And Slade's biggest name he has is basically Prometheus, Dr. Destiny, and Toyman (Hiro). An argument could be made for Count Vertigo, but everyone else is pretty low tier on the list, heck I didn't even know who Dr. Moon was.