According to the absolute majority of evidence and history, an animal called a human is the best animal for other humans to have sex with.
Other than that birds and kittens are commonly regarded as cute and often make a woman's heart melt.
The answers you are going to get here are absolutely predictable and will be these exactly and exactly in this ratio:
1. Dogs.
2. Horses.
3. Boars.
1. Dogs are the easiest animal to have and to have sex with. Also the most common in porn people want to watch.
2. Horses are the second most sough for in porn but more difficult to keep. But people want that content and will attempt to persuade you to make it for them.
3. Even less common but for some reason also regarded as hot by a significant amount of people.
Other animals are either hard to have or unsuitable for sex or not interesting for the majority of porn watchers.