The boss in maw oh lorkhaj always seemed super strong to me. Maybe a dragon.

Lorewise? Arguably one of the daedric princes we fight. Even if the fights are fairly easy because it's zone content we tend to have some sort of supernatural aid from the aedra or other daedra at the time.

That's actually pretty hard to judge as a mere scamp have almost no difference to daedroth in terms of power budget.

If I'd go just from lore alone, I'd say dragons overall. Obviously I won't say that with Skyrim's or ESO dragons depictions in a slightest. Also Rada al-Saran seems like a pretty formidable foe.

daedric princes > dragons > celestials > rada imo

Rada al-Saran hands down, I think for ESO bosses, I could be wrong but I don't think I am, its disappointing his boss fight wasn't comparable to like Molag Bal's boss fight. The reason why Rada is the most powerful lorewise, and he would certainly be more powerful then I would say most of the dragons in ESO's content. The reason is his skill, and his power at Sword Singing, Before he became a powerful vampire lord, he was one of the most powerful mortals n Tamriel, powerful enough to take on a god for three days and three nights, and neither side could win Rada won that fight in my eyes though as he proved himself., he didn't see it that way. And this god is called Leki who is on MKs' one of the writers whos most known for helping do most of the deep lore for TES. More powerful then Auriel in the list of more powerful beings according to him. So Rada can beat out the Avatar of Akatosh technically going by that logic and the lore we know about him. Something even Dagon himself couldn't do because power level wise, Leki is more powerful then Akatosh. That is going by Mk's list of most powerful beings. Now he was still mortal at the time but the fact a mortal could prove himself capable of nearly winning against a god, speaks for itself.

He was capable of using his sword singing though I think the vestige was kind of immune to a lot of it given their daedric and prisoner hero nature. What makes Rada still interesting is he could kill anyone he wanted nothing could stop him except for like powerful forces/prisone heroes I believe, he could go like full Arnold Schwarzenegger's Commando movie stuff, where he like takes armies out all by himself and likely be able to do it without being a vampire/vampire lord.. He could have single handily destroyed all of Skyrim with one sword stroke using the forbidden technique which he knows he knows the tenique that destroyed all of Yukuda. So basically Rada has contiant destroying powers, if he wante d to destroy skyrim he could have, instead he choose not to, likely not wanting to destroy everything. He just wanted to set his comrades from from COldharbour. Only doing what was needed to break them out. He was very likely the most powerful vampire lord outside of that he has full bat wings nand clearly was far better and far more powerful then Harkon would ever be in his. HE was also far more impressive. He was very mighty in that form too, but what he was most powerful with is his humanoid form. That is what makes him one of the most powerful bosses lorewise. All it takes is one sword stroke of the bobbiden technique and a large portion of Tamriel would be destroyed by his hands.

Rada-al-Saran since he fought a goddess as equal.