If you follow the in-game mechanics... Babette. She's a Master of Alchemy, capable of creating poisons that can kill just about anyone, or potions to fortify her abilities to the extreme... and she can't be attacked or damaged in any way without mods.
In all seriousness, the leader(s) of the Psijic Order are probably powerful enough to wipe out entire cities if they needed to, and gaining the Eye of Magnus has no doubt only increased the resources at their disposal. For all we know they may have enough arcane power to nuke a city, or worse. We know for a fact that at least three of them can both teleport and stop time - with that amount of power no person could possibly be safe from them. Say what you will about the power and reach of the Dark Brotherhood, the Psijics could likely murder anyone in Tamriel if they actually felt like it... and they could make it look like an accident.
You know what's truly frightening? Chances are they already have.