I am a cicada. Someday I will stop being an underground crawly cicada, and will shed my skin and grow wings and make lots of irritating noise. I would like to be megatherium. I want to know whether they mostly clamber around the forest floor, or if they swim around more than people think.
I would not like to be a horse. To be that intelligent and be made to stand around in a stall all day... no wonder they self injure if they're bored. You don't get to exercise all day and then a stinky, bossy human sits on you and makes you do tricks. You can't brush your own hair, because you only have hooves, and flies bite you.
I am also glad I'm not a koala. Those guys are total jerks, and all they ever eat ewwcalyptus. I bet they used to eat other things before the desertification of Australia, but that all the tasty stuff died out.
You can see with only one eye open, but you'll probably run into things and stub your toe. The big picture matters.
I think I would be a beaver. A giant, pleistocene-era beaver. I would make ponds in the boreal to rival the Great Lakes, a project suitable for a giant permie.
I would want to be a woolly mammoth, managing the new grasslands north of the treeline. Just another keystone species stabilising an unstable, disturbed biome.
I don't think I would ever want to be that breed of homo sapiens that embraces thoughtlessness. Willful ignorance for short-term personal gain has got to be a giant, glaring road sign warning of an evolutionary U-turn.
EDIT: If the question were changed to encompass the other kingdoms of life, I would want to be an Ent.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein