I have to say cats..
Don't get me wrong I love dogs and just about any other animals out there. But the relationship between a dog and it's master is just that, your his master. His affection comes from a dependence on you.
On the other hand, a cat is independent. Cats don't need us, they come and go as they please (in some homes) and almost always return. With a cat, you are equals, so when a cat shows affection, it's genuine...