Just curious what is the star attraction for most folks (you can only choose one). Personally I'd go with the monkeys/primates myself although last time we went to the San Diego Zoo we saw a camel fellating a gazelle which was pretty damn hilarious...
My first instinct is jungle cats, but truth is, while they are awesome animals, they can be quite boring to watch as they spend a large portion of the day asleep. The African animals are just so diverse and exotic, they are a lot of fun to observe.
Gorillas all the way. Although a lot of zoos don't have them. Fort Worth does, iirc, but the Brownsville zoo has the best gorilla exhibit anywhere. Pretty amazing.
The primates. The gorilla habitat at the Bronx Zoo especially around feeding time is second to none. It's truly amazing to watch them. Also The Gibbons are great at just about every zoo I've been too. They are playful and love to swing.
The primates are the best. <iframe title="YouTube video player" width="640" height="390" src="http://www.youtube.com/embed/HBUKRFXGuqo" frameborder="0" allowfullscreen></iframe> Also, this one is funny although maybe NSFW: Spoiler <iframe title="YouTube video player" width="480" height="390" src="http://www.youtube.com/embed/khwjD-KVQ_Q" frameborder="0" allowfullscreen></iframe>
apes. i used to have a chimpanzee that my parents forced me to donate to the zoo. They got me a wistiti monkey as consolation...ended feeling more like a punishment.
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i think it goes beyond the restricting category of just reptile. I present Blanco. Spoiler some clarification on why he isn't albino.