Umm... uh... y'mean if I could *look* like any celeb? 'Cause, I gotta say, most of them, I'd probably not want their lives, fer real.
Honestly though: if I could have been anyone at all? Emma Thompson. Wonderful person, stunningly beautiful during her youth (and still looks great at 55, thanks much) and the epitome of grace and poise during some pretty horrible crap.