In my head, Demise is the strongest big bad in Zelda. He was also the 1st big bad.
He is basically "The Devil" of the Zelda Franchise. He is the source of all evil/monsters/final bosses.
Again, this is just in my head and just for fun. Its not something Im going to "seriously" debate.
If Demise and Ganon/dorf were video games, Demise would be the base game and Ganon/dorf would be a DLC. Or, Demise would be a "mainline" game like SS/OoT/BotW and Ganon/dorf would be a spin off that doesnt even have the words "The Legend of Zelda" in the title, like Age of Calamity.