The strongest character I have in my stories posted here (aside from one story), is Yellow, aka the True God of Esclakpar. She's an entity who can, and has, killed multiple Cthulhu and above classed Eldritch beings with ease in the past. Pretty much she's a God who rules over many other Gods and just has to make sure nothing goes wrong and, if something does, step in and see that it is fixed.
To put it simply, there are four rings. One ring is the common Gods and etc, as well as the one our world prays to. The next ring is Eldritch beings or Supreme Gods. The ring above that is made of the toppest of Eldritch beings and Supreme Gods, as well as young and new True Gods. The final ring is True Gods, or Trunaricis Esperdaris, beings who are so overpowered yet lazy, as well as few and unique, that all of them are just crazy and out of the norm. (In her past time, Yellow literally cosplays as an .EXE out of pure boredom)
However, she's really just a psycho who loves to kill people and see them struggle and improve, often doing it sadistically. Despite all the powers at her fingertips, she'll hold herself back to the point of even risk being put into hibernation from being wounded fatally just so she doesn't get bored.
The other character, and one I can say without spoilers, is Ai, the self-proclaimed and future Apostle of War. She is the most powerful Wartorn, and at her peak of her rage and determination to get stronger and win, fought against the Apostle Aven, almost to a standstill with Aven not actually holding back. To reference, a God over on that world has enough power to destroy galaxies, and even most of them cannot stand against the pure might of an Apostle, who rules over them.
If one really wants to know the most overpowered and, if pushed, also overpowered characters I have on this site, I'll wait till someone asks since I'm cheeky and lazy.