[Kiwami]

I’d say Roidmudes, Inves, Imagin and Worm, because those go straight for world domination. The Roidmudes want to destroy/dominate mankind, the Inves/Helheim will eventually devour our world if not stopped, the Imagin want to destroy our time to create one were they rule (something like that, the Imagin and their leader are so pointless I that don’t even remember) and the Worm, well... they want world domination too, although it’s a bit more complicated and too spoiler-heavy to explain but people who watched Kabuto will get it.

I cannot speak for the Zodiarts, Undead or Orphenochs since I haven’t watched the shows they appear in so I'll just assume they are not much more dangerous to the whole world than the 4 above.

The others are threatening as well, some much more scary, but they don’t go for world destruction/domination so they are not the most dangerous threat to humanity as a whole. Mirror monsters are freaking scary, but they only eat people every so often that the world apparently doesn’t even realize they exist; their victims are just filed under "vanished". If they would be as lethal as the Worms for example you would think that humanity might have already figured out their existence.

And some are just a joke, like the Phantoms. Aside from the Sabbath I don’t think they manage to create a single Phantom in the entire show, and even if they did: They’re entirely useless as they only serve one goal, and they don’t even know about it.