Mice...just mice in general, they don't look right in-game (With the exception of Rod, Bree, and Chadder, they're adorable).
This is why fanart is the best thing to ever happen in ACNL, because villagers look better with that artist's artstyle. (Pretty much for any villager IMO).
I love cats in real life but haven't been able to make a connections with ANY of the cats in game, and I have had a few that I thought I would like but didn't. Their base is fine but I don't really like the personalities or how they look most of them. =[ I really want to try to get Ankha so she can be awesome and make me like in game cats again...
For me, it's koalas. I like real koalas, but the ones in Animal Crossing just annoy me for some reason. Melba is cute, and Sydney is okay, but I don't like the others at all.
Tigers and lions of course, lions especially. There's not a single lion in AC that I actually like, and there's one that actively gives me nightmares.
The worst has to be the alligators though. Alligators are my favorite animal IRL, but in the game they look so hideous. It's probably the position of the eyes. If their faces looked more like wolves or something I feel like I'd like them a lot more.
Some of the tigers are cool. Claudia, Rolf and Bianca are my favourites. I have to agree that the lion model is awful, however. A lot of the mice look strange to me, and in my opinion the kangaroo models are horrible.
I really dislike the lions/tigers, the monkeys, the gorillas, eagles, kangaroos....and chickens.
Frankly, though, I don't particularly like kangaroos or chickens in real life.