a tree, standing in the countryside, in the wind, rain and sunshine, providing shelter and photosynthesizing to remove carbon dioxide and produce oxygen. I would then be providing oxygen for breathing and growing wood for people to use for cooking, building and so on.
I'd come back as one of my dogs as they seem to live the high life. Or a tortoise, 226 years I think the oldest one found was. Either life slows down for them and everything seems fast on the outside. Or they have a secret that we all don't know about.