I know a lot of people have named animals on the list, but I wonder if being a human counts? I'd rather stay as a human being or if I could, try out a new life as the opposite gender for an entirely different experience. Humanity has quite a dark side, but I've grown quite attached to the good sides of it. I cannot imagine what it'd be like to be a bird, since they are on a different cognitive level and age-span.
Humans aside, I'd probably pick the cat. I've grown a liking to cats for a while.