@cwenwyn: Can we make this for all zoos so it's not just me and you having a conversation? Lol.
And to answer your question: I'm there almost everyday so everyday is good day to visit. Depending on the animal and your personal preferences determines the "best" time - this is applicable for all zoos. Obviously the best time for aquariums, the Reptile Building, Indoor Asia Quest, and the Roadhouse is winter time during the day. It's not as crowded and the animals are active because they are not affected by the weather. Spring and Autumn are the best times for North America because of the new or dying foliage and all the animals are out and active. For the rest of the zoo, summer morning or late at night is the best time because animals are active at these times and it's not as crowded. Also, most animals take naps or are transferred around during the afternoon so they are visible or aren't as entertaining.