OK so “Die süßesten Popo” is a weird mess and there would be several correct options to go in, depending on what you were going for. “Der süßeste Popo” would be the correct singular, “Die süßesten Popos” would be Plural, if you were trying to go for what you would use in a sentence about having it (aka Dativ) it would be “Den süßesten Popo”. I hope this helps :) I think you got confused with the German gender thing tbh, because “Popo” is male for some reason.