If you are trying to get the "Complete Animal Species Holder" achievement, I would recommend the map 16. "Congo basin". "Krüger National Park" (17) and "Arizona" (13) are bigger but they have one big downside: On these maps it never rains ... due to that fact you would need considerably more gardeners to keep the animals totally satisfied. (If you only want to get the trophie you can get away with not using gardeners at all. Just get all animals in the zoo and put enough plants in the habitat. They will probably wither now and then ... but when it rains all plants will get water at the same time ... so all animals that were unhappy because of unsufficiant plant coverage will get satisfied at once wich will trigger the achivement (of course only if all other needs of the animals are met)