My favorite is Poppy!! I had her in my original New Leaf town and my original New Horizons town. I love her design and she's always been my chill villager in a town of grumpy old men, her interactions were always so funny. I also love Hazel. Next are Filbert and Mint, they were besties in my city folk town. I don't have any opinions on anyone else, I've only also had Agent S who's okay but I don't have strong feelings on him either way.
My top three are Agent S, Static, and Peanut. (Mint used to be my third favorite, but Peanut grew on me when I had her in my Wild World town semi-recently.)
My least favorite would probably be Hazel or Ricky...although I'm not fond of the Splatoon squirrels either.