What is the most powerful character you've played with in your tabletop sessions (as GM or player)? Ever had player achieve demi/godhood, or meet superpowered NPCs, gods or opponents? What are your tips or advice on making these encounters interesting or as fun/fair as possible?

>D&D: probably Demogorgon or Vecna. Not sure which is stronger, considering their mooks and holdings.
>COC (modern): Baba Yaga from "Inversion". Very fun scenario to run and play with tons of excellent resources, though requires a bit of effort from the keeper to prepare and set up. Without spoilers: its a Soviet-era science bunker full of increasingly spooky monsters, its a 200+ page book with incredible details, so detailed we played it twice (first run was a TPK). Highly recommend.
>Traveler: homebrew !Not_Xeelee ancient ayy lmaos PCs discovered hibernating on a remote planet waiting for the heat-death of the universe. PC meddling woke them up, which lead to bad stuff, though that was more incidental than deliberately malicious akin (like an awakening Necron tombship), like sweeping off the snow and dust off their parked car as they went about to continue their agenda.