I have two favorites: Poppy & Nana. Poppy because I just love her to death and Nana for a more sentimental reason.
My grandmother was always called 'Nana' and when she passed away, I was devastated. I got City Folk for my birthday that year and I was really really upset over it being my first birthday without my Nana. I booted the game up though, because my mom and I were anxious to see it after having only ever played the game cube one and guess who one of my first villagers was?

Nana! Just made my birthday for me. So I have to have Nana in every game I have for AC. I don't think I could ever let her leave.
I think my least favorite villager is Violet. I've always hated her, ever since City Folk. :3 Remember just following her around, bopping her with a net to try and make her leave lol