Humans - all other animals make their life more fun. However, if I had to pin it to what we determine to be animals other than human, I would say dogs. I love dogs, they are loyal, faithful, love unconditionally, and generally make us feel more, well human. When properly looked after they have the life of Riley, and it shows in their persona.
Of course, it may be that I love dogs and appreciate the friendship they offer up. Had three in my life who lived their full lifespan, so had them for 50 years of my life and so sad that I can't have another because of my inability to looked after a dog properly. If I could, I'd have a Beagle