What is the easiest group of animal to photograph?

I find it depends on the exhibit the animal is in. Getting good penguin photos can be easy in some exhibits, but in some glass-fronted exhibits the water droplets on the glass can make photography more difficult. Likewise, a lot of species can be great to photograph in some exhibits, but in other exhibits the lighting can make things a lot more difficult.

It also depends on your camera. My camera does very bad under low lighting, while my phone does horrible whenever you need to zoom. So while I've gotten good photos using both, there are circumstances where only one or the other could yield a photograph.

 

Great apes. They almost work with you for photos

 

I agree completely that a good photograph often hinges on the exhibit. Unfortunately, my local zoo has many animals behind wire enclosures, which often wreck an otherwise fantastic photo. I've captured great images of mandrills, Coquerel's sifakas, howler monkeys, etc. looking directly into my lens, but the darn wires significantly impair the photos. It is very frustrating. As you mentioned, the glass exhibits pose their own challenges with dirt, grime, and reflections. Sometimes a lack of simple maintenance is also an issue. Last week I went to the zoo and found the caiman lizard in the best pose I've ever seen it, but the exhibit light was out rendering it so dark it was useless to try and take a photo. There is really no excuse for burnt out lights in reptile exhibits!