I want to be a cat, specifically a house cat that is treated as well as mine are. They get the primo spots on the bed, food, treats, attention when they want it, and someone else cleans their bathroom.
But....I spent several years doing animal research, and if there is such thing as karma and reincarnation I think I've earned several run throughs as a experimental rodent. Probably a little mouse with graft v. host disease, or with a nasty little T-cell issue. But I'd much rather be a house cat.
Second choice would be an eagle, because I would love to be able to fly like that. Don't know that I'd like a steady diet of rotting salmon though.