Another cliche answer, but mine is Dogs.

There is nothing in this world, like a dog. I've never met one I didn't like. They don't care how you look, if your breath stinks, if you're rich or poor, they accept you for who you are. Every week when I walk into the shelter, and see all the homeless dogs, I can't but wonder why such a perfect creature has to suffer like this. They forgive us for making mistakes, and still love us unconditionally. The way they wag their tails when you come home from a long day, the way they drop the tennis ball into your lap, their desire to please, the different jobs they have to help humans, (EX: Guide Dogs, Search and Rescue, Service Dogs) I could go on and on. They're perfect.