The most dangerous? Izanami of course. She couldn't be killed through normal means and the Time Killer, she was a powerful Observer, and Susano'o is just Terumi in his armor. Terumi was the most evil, but Izanami definitely was the most dangerous.
I guess that really depends on what you argue as most dangerous. I don't think Izanami is the most dangerous mainly due to her indifference to most things, and her ideal is the make the world end. Terumi on the other hand doesn't think twice before getting his hands dirty with whomever. His goal puts him on the more dangerous side too, opting to continuously destroy the world over and over until he's satisfied.
That, and at the end of Central Fiction he fused with Noel, so wouldn't that mean he took in Izanami as well?
But him fusing with Noel didn't gave him Noel's powers, or else he would just deactivate the Master Unit and watch the unobserved world being erased.
And Izanami and Susano'o have similar objectives, just, Izanami wants to destroy one world, Susano'o wants to destroy more worlds, so he would be feared, which would make him stronger than Amaterasu, so he would be able to break free from her control. His goal, freedom, doesn't seem as bad as Izanami's "destroy the world just because".
Overall, I'd say that Susano'o was the most destructive and overall threatening of the BlazBlue villains, followed by Izanami and then Relius at a distant third.
The biggest factors Susano'o held in his favour over Izanami is in terms of overall combat ability and scope of his evil plan.
While Izanami boasted (not without justification though) that she was unkillable, that was moreso due to the fact that it's more like fighting the Master Unit's angry puppet rather than her being unstoppable in battle. Susano'o almost effortlessly crushed everything in his path once he got his real body back, and he only lost because Ragna had literally the only power that could defeat him (ripping out his soul/Terumi) from his body (the Susano'o unit). For all intents and purposes, he was unstoppable unless he was removed from the Susano'o unit, the same way Izanami can be defeated but she'll never "die" as long as her originator is around to bring her back. Her main advantage was being an Observer, which Susano'o also matchched by absorbing Noel into himself (which raises the possibility he could've done the same to Izanami instead if she had won the Observation battle with Noel).
Susano'o was aware of the Gate leading to the Azure and how to obtain it, how to cancel out even Amaterasu's Phenomona Intervention (by absorbing Noel), and even how to escape Izanami's Doomsday plan. Once Izanami had her Doomsday set to go, she basically just focused on buying time, and her boasts of finding the true Azure ring somewhat hollow when she made no efforts to reach the Gate herself or to get Ragna to the Gate. She was only interested in spreading death.
Even if she did complete her "world of death" master plan, in the long run it's considerably more merciful than Susano'o's master plan of destroying the world then reviving it over and over to get as much hate/fear out of them as possible. By "free", he simply meant that he didn't want to be beholden to Amaterasu's wishes through Phenomona Intervention anymore, and basically wanted to vent out his spite on Amaterasu by destroying every world she reconstructed.
Izanami actually does have a reason for destroying the world, which amounts to the equally petty and "doesn't seem as bad" reason of destroying the world because her original self was tortured and broken by humanity. In summary, the two most evil characters in the BB verse tried to destroy the world because they wanted revenge on everything for their misfortunes.
In conclusion, both were pretty despicable and very competent villains, but my view is Susano'o had the more solid plan and was the more dangerous enemy due to his initiative and sheer power.
Susanoo/Terumi. It's not just about holding an immense power, otherwise Relius Clover would be on here, since I always have to wonder whenever he smiles whenever he's defeated or the fact he can recreate a lot of things including Takemikazuchi just by looking at it, just.... How strong is he? I mean, he's been revealed in his dialogue with Nine that he was One from the earlier Ten Sages. And yet, he's not the most dangerous at all since he doesn't really care what goes on around him or interjects much in it. Anything that he does must have some interest for him to somewhat initiate and follow through. And from his track records so far, he just merely captures/beats up people and experiments on them if it happens to pique his interest. So it's all about the passion and desire in wanting to become the most dire threat, is what makes a person very dangerous. So going back to Susanoo/Terumi, he fits that exact bill. He wanted the true Blazblue so that he could create a world of hell. I mean, Izanami's purpose was just to bring death to the world and was just an extension of Noel's disappointment and sorrow against the world, so we all know what's her deal. And honestly, death and perm silence is much better than an eternal hell of suffering.
Because what I don't remember is, even though I just finished the CF game, is that did they ever mention why he wants to change the world besides wanting to escape Takamagahara's and Amaterasu's influence? Or why he wants to make people suffer and fear him?
@Kentaku He does ring a bell in somewhat being very dangerous due the fact that he can even bend/influence space time with his pure strength and will. But just pure strength doesn't mean anything if he doesn't understand the things around him. Like you said, he just doesn't have the mind for it, which is why he got beaten by both Kagura and Jin kinda pretty easily. That and no matter what your stance is, pure physical strength just does not beat any higher power such as magic or god-like powers. Besides, they never really pit him against the gods or even the other villains (besides Izanami), and was always set loose upon the "weak" unsuspecting heroes all the time. So we can't even truly gauge just how qualified he is to stand among the likes of Izanami, Terumi/Susanoo or even Relius Clover. Besides, he just wants a good fight. No different than a single-minded street brawler or gangster (which was most likely the influence for his design).