Definitely not a human. More than anything, I'd want to be a bona fide ghost so I could haunt everyone who did me wrong.
After that loses it's thrill I'd want to act as a guardian angel of sorts. Helping people out, keeping lonely/scared kids company, bringing lost pets home.
But I'd want the option to expire and dissipate when I choose. I can't imagine having to exist in any form for an indefinite amount of time.