Maybe a horse: a domesticated one, that is. Nice and easy life (food and protection from predators provided), but you still do some work (being ridden). Wouldn't want to sit around all the time. Plus, it's a very athletic form to be in
Although it would depend on what your owner was like.
Or maybe a bird of prey, because it would be fun. Or, like others have said, a Yeerk, if you were assured a supply of voluntary hosts, because then you still kind of have the opportunity to change form, even as a nothlit.