Margarita. Aside from maybe Gallerian she is the most intelligent of the sinners. She was able to refine her poison, turn it into a plague and test it out on the townspeople without anyone (aside from Elluka) having a clue. Also that dress of hers is amazing.
I have very many. But I consistently love Yarera and Zusco because they were so hilariously slimy and incompetent. And yet they were trained in niche assassination techniques and one of them actually started a family with someone >----> Just loved seeing the way more sensible and decent characters played off them.
Then they became a running gag through their descendants and it comes to a wonderful head in the Judgment novel.
Sad I will probably never see what they actually look like from the DoE manga.