Ozzie would be the one.
Tabby and Katt.
Umm Peanut, Hugh, Gala, Kid-cat, Mira
I think Mira and Kid-Cat and semi-popular though.
Katt and Wendy are probably my most unpopular. Though I've really noticed Wendy getting a lot more attention lately...same with Katt but nothing too monumental.
I hope to find Bill eventually in my second town since he's my favorite from gamecube days....so
I guess he'd be pretty unpopular as well.