This is open for much debate Vivec or Vehk is very powerful having obtained CHIM.
There is the Daedric Lords powerful beings with exceptional power.
Lord Nerevar reborn would be powerful having gained something like godhood after becoming afflicted with corprus and getting all of the positive effects and none of the negative and then there is his influence in Morrowind but that may have faded some these past few centuries.
Tiber Septim united all of Tamrial and he might have obtained CHIM as well.
The above post in spot on to Magnus is the very being that causes the vast amount of magic on Nirn to exist and he had great power before the creation began. Also like Commandant Oreo pointed out all magic comes threw the whole he ripped in the void from Atherius, my scholar is Andoral is very interested in Magnus as well and he is one of the most interesting topics in TES lore.
Sithis is the dark void and he has a group that kills for him he is the dark lusting after your very life and his powers are great but by the most part is unknown.

Wabbajacking him? I'm confused, that works? No.... I fought him like I would fight anything else. It was really sad... that fight only lasted several seconds. I used a spell that did 60 shock damage with 100% weakness to shock and 100% weakness to magic.... stacking the damage till I was doing several hundred damage per cast. Dagon then collapsed into a puddle. I consider that to be 'defeating' him. So he gets back up... that is the perk of being a deadra prince. And then Martin defeated him in a longer lasting way.

Magnus was around for part of the creation of Nirn but escaped before he lost his power. His escape created the sun and the hole to Aetherius. At least that's my theory.

I never said it was their intended goal, it could just be an unintentional consequence to their actions. Maybe the ban was an attempt to remove their greatest enemy's power, leaving Talos vulnerable. The elves blame the trickster god for the loss of their divinity. The majority of Tamriel wouldn't oppose the concordat too heavily, as the majority of them lack and specific reason to worship Talos, other than being told to. The Nord's see him as one of their greatest heroes, so it goes beyond faith into reverence for them.
thinking more of a possible plot than realistically
If CHIM is involved, mortality holds less of a grasp on those that are... awakened. During the Sermons, Vivec actually hinted that you could kill him to gain a shortcut through the quest. Mentioning that he will slay you again and again until you succeed (you reload over and over)
Aside from that, Vivec didn't hold infinite power, more that his power was particularly potent. Vivec has banished a daedric lord on his own.
All in all though, Vivec didn't seem particularly interested in his own life or death. A side effect of the whole CHIM thing. The reason its not used for evil, is simply because those that can use it seem to lose all ambition, after realizing that everything exists between the is, and the is not, -1 and 1. As such, everything is zero or nothing, and all their actions actually have no end. (right around here is where most of those with CHIM remove themselves from existence. They do not, and never have or will exist. A select few, faced with this absolute truth seem to accept it completely, and deny it completely. Knowing they are nothing and everything, but still referring to themselves as -I-.
Though, speaking philosophically, doesn't that make sense? If one were to ask god its name, wouldn't it refer to itself as its name (or "I")?

If I were to try and rank the beings of power.
1. Godhead
2. CHIM Users (being they are (regardless of how small) an aware part of the godhead)
3. Padomay (Sithis) and Anu
4. Magnus (Aedra who hasn't given up his power)
5a. The Daedric Lords.
5b. All the remaining Aedra seem to hold much higher individual power but also face mortality.
6. All lesser gods

Anu, or Anuiel, or Auriel/Akatosh, depending on what your definition of "being" is.

By ripping into the Ather and giving us all our wonderful magic.

"Vivec: "How does it feel, Lord Azura? To so fully manifest here is the Mundus, stripped down only to your name? Perhaps it feels a bit like my sister did, when your machinations split her, name from land, nymic eth maliache velot, thoughtless save for domain. AE ALTADOON DUNMERI for my sister's madness I eat you." "
Would you mind explaining this part to me? What is Vivec talking about when he says his sister was split "name from land"?

Vivec's dialogue after Azura explodes also seems...off to me.