In my opinion, Riven would be at the top. Since Ahamkara can transcend death in terms of whispering (Through lore from Ahamkara armour) they can basically live on no matter what and they never really die. Very good question. 2. Atheon, who can literally fold time and trap you for eternity inside a giant vex place, would probably be second on my list. Oryx, with his throne world as such would be third. He might honestly be stronger than Atheon. Because in terms of lore, he literally governs the laws in his Throne world (Which is why he's so big.) so he probably can govern how strong you are, or if you're even alive. May be going insane, lol. 4, Calus. Since we know, (or maybe I dont know) all the powers Calus has like his little Void realm (I dont believe there's a name for it) and the other cosmic powers he refers to 'existing beyond our feeble light'. 5, would be Crota. He's still particularly strong (of course enhanced by the laws of his throne world). He didn't really show us much outside of his connection to Eris and Oryx, he probably could've done much more. Only reason we defeated him was by exploiting his own sword. 6, at last would be Aksis. As you said, he is an old kell enhanced by Siva, but still powerful enough to knock out an entire fireteam of guardians. But he still ended up exploited like crota, using the Siva charges his enhanced servitors had at their cores. A very interesting topic, though I haven't really read that much into the lore. I mostly watched videos about it, and read half of the Books of Sorrow. The games lore is fascinating, so it really has piqued my interest and I feel like reading everything due to that fascination.