Tangy is my favorite! Then Kiki, and then Olivia. I also think Lolly is really cute, but I've never had her in my town so I have no personal attachment to her.
Honestly though, I think most of the cat villagers are adorable~
Rudy just made me mad, I once had Ankha and she was nice to have around i guess but.. none of the cat villagers really made me too happy aside from tangy