Sign-language monkey every time. Imagine being able to have a conversation! I know there are primates that actually do this too. It's now my #1 aim in life to meet a sign language monkey!
Monkeys aren't good pets, though. They can live happily in zoos and such where they have other monkeys to interact with, or in environments where they have many qualified humans to interact with, but you're just one man. You wouldn't have the time to provide proper care and entertainment for a monkey, particularly one intelligent enough to learn sign language.
It's funny but I never actually thought it through like that. I was actually thinking of some footage I saw of an anthropologist who taught a monkey sign language in a zoo setting. I guess I was caught up with the idea of proper communication with another species.
Why? Because fuck yeah. That's why.
I pick dog