I'm going to list the options you gave, starting from worst to best:
Monkeys: Since there are only 8 monkeys in New Leaf, you will have to constantly deal with another species moving into your town. That might get annoying very fast. There are also no smug monkeys if that personality appeals to you. I suppose you could make it work, but you would probably have to add a couple gorillas (which wouldn't be a single species town anymore).
Koalas: Better than the monkeys, but still only has 9 villagers. With that amount, as long as you don't pick up villagers from anyone's void, you will not have to deal with move-ins. I do not believe there are peppy koalas in New Leaf if you like that personality.
Rabbits/Ducks: I'm putting these equally because they are both very good choices, and which one you choose really comes down to personal preference. Both species has over 10 villagers, and they each have a villager in every personality. Personally, I slightly prefer rabbit villagers but that's just my opinion.