Mammals are near the top. They have longings and desires beyond survival. Even the lowest of mammals has a fairly advanced memory, and play is a large part of the development process.
Humans are obviously the top of this pyramid, but I believe there is a sub-step before humans I guess. These animals have fairly advanced cognitive abilities and memories. Primates, dolphins, elephants, and pigs are the most advanced, with mammals that have been domesticated for thousands of generations (cats and dogs) receiving honorable status.
There are some interesting things about pigs that most people would be surprised to know. There really isn't another animal that can make as great of a pet one generation in to domestication-- even a wolf can be extremely difficult to train. Additionally, pigs are one of the few animals, along with primates and dolphins, that experience sexual pleasures and desires beyond reproductive purposes (farm jokes aside...)
My purpose with this thread was not to make people feel guilty, but to just look at the topic from a different light. It's sort of the "omnivore's dilemma", where mercy and primal desires come to heads.