My favorite monkey is probably Elise because of the glasses she wears and the exaggerated lipstick and beauty mark she has, because I prefer villagers who have unique aspects to their designs that set them apart from the rest of the animals.
Honorable mention goes to Tammi because I think her red eyes look neat.
I like Elise the best of the monkeys because she's probably the most unique, save Shari (who is a little too realistic for me). I'm not a huge fan of the monkey villagers, but I know that's 100% because I started my town with Flip, I don't like his design, he's a jerk, and he won't move away. x_x
I don't really like the monkey villagers as a species, although when Tammi moved into my City Folk town she grew on me quite a bit and I actually liked her, so I voted for her. I don't like any of the others, though.
Shari is my opinion, best monkey. Her design is nice. Her personality works on her. Chuga's lets play (and Pansear-and-Nana's depiction) adds more depth to her and makes her even a much more better character. I seriously want her in my second town now. ><