If I had to choose? ... Hrm.

As far as gods go, I'd probably have to give it to Talos as much as I'd like to say the Aka-Tusk. Simply put, Talos is the replacement of a dead god (see: Lorkhan) who achieved divinity through multiple pathways: he mantled the original enantiomorph between Tiber Septim, Zurin Arctus, and Wulfhearth and created a Tower to fortify the Mundus, he achieved CHIM (see: "From the Many Headed Talos", Heimskr), and united the Empire by reactivating Walk Brass, which I'll get to later.

Discussing deities gets murky, because the Aedra - though diminished - had a great deal of impact on the Mundus. The different Daedric Princes all have constantly changing waters of Oblivion and none of them have been "removed" by the others yet, so I'd assume that within their own domain they're pretty much unstoppable barring an intervention from literally the entire Pantheon. The kalpic cycle further complicates this, as whenever Alduin eats the world, everything goes back to Convention (where Lorkhan gets killed by Auri-El and his heart is shot into Red Mountain) and the Aedra and Daedra all play musical chairs and switch places. It's easier to think of the divines as "ideas" or concepts that are preeminent in the Aurbis, and considering that they're all more or less equally powerful because A) if there's an "idea" that isn't represented in the Pantheon, it's probably not that important and B) in some way or another, the strength of a god is dependent on how fervently they are worshipped. This is where you see examples of "dead gods" like Alkosh springing to life to backslap the murderous Pelinal Whitestrake from continuing his pogroms into Elsweyr.

But if I had to pick the most dangerous entity in TES? The Numidium. Hands down.

The Dwemer saw the Aurbis for what it was: a prison. Same as Lorkhan, who saw the name of God - "I" - and tricked the Aedra into building the Mundus and subgradiate far enough down that someone might find a way to escape. Lorkhan's plan was to keep going further down (see: Anuiel -> Aka-Tusk -> Auri-El -> Alduin & Akatosh & Alkosh etc.), until mortals could find a way to reconcile the fact that they were part of the Godhead's dream and escape. His hope was that people would see the universe ("I AM"), renounce it ("I AM NOT"), and manage to not get zero-summed out of existence. Those who do achieve CHIM. Those willing to go the next step - becoming a new Dreamer - achieve Amaranth.

But no, the Dwemer were not interested in this. Their plan was one of antegradience: unify the entirety of the Dwemer into one singular being, one God, and move back up the ladder until they reach the starting point. To do this, Kagrenac build a massive machine, Walk Brass, the golden-skinned golem that would serve as the ultimate World Refusal. The Numidium is the Anti-Metaphysic personified. He causes the Warp in the West and the Red Moment. His activation in Rimmen by Tiber Septim at the end of the Interregnum created a temporal distortion that destroyed the Halls of Colossus and irradiated the surrounding area for centuries. His fight with the Altmer is still going, as he fights their mathemagicians well into the Fifth Era in alternate dimensions. He is destruction incarnate and a weapon the likes of which has never existed on Nirn. You know how the Yokudans can blow up continents? Numidium can blow up planets. He exists to destroy the Aurbis, and thereby reject it entirely. He's the kind of crazy you just don't mess with.