I know people consider him overrated, but my all-time favourite is Bob. I made a detailed explanation at this page:
http://www.belltreeforums.com/showthread.php?397068-Who-is-your-favorite-Villager/page3. Majority of my favourite characters are also cats and here's a list and why in no order (excluding Punchy, he's really cool though I've never met him):
Tangy: Appeared in both of my GCN towns and is currently in my New Leaf town. She is one of my dream villagers and she has a unique design.
Rosie: Really cute cat who appeared in my second GCN town. She also appeared in the movie (which was excellent) and was one of my favourite characters in it. I personally prefer her movie design over her game design more though, given the better looking eyes. As of the time I posted this, I had a signature that features Rosie from the movie. I slightly prefer her over Tangy, but not by that much.
Mitzi: My first ever normal villager in both of my GCN towns. She is my favourite normal villager and she was my favourite character before I knew more about Bob. I remember when she told me to plant flowers around her house and those flowers are still there (I don't plan on getting rid of them). She did move out of my second town however she eventually moved back in, on the same spot too.
Rover: I can't decide if Rover or Tom Nook is my favourite special NPC. Whatever. I think Rover is slightly underrated, yes a lot of people like him but whenever a big YouTuber plays an Animal Crossing game they always seem to hate on Rover and I feel bad for him. He's not a stalker, he's just a train (and maybe bus) enthusiast. There's a reason why I don't like Wild World as much as the other games: Rover barely appears in it. He only appears when at the Roost or setting up multiplayer from what I've heard. Had he been given a bigger role, he could've appeared in the movie (as it was based on Wild World) and that would've been awesome. One day I hope he will become a smug or lazy villager, even though I'd probably want Bob as my lazy villager more unless I move out another villager type.