I've given this so much thought in the last few years because I mean... Whatever why not? My answer is the Big Bang episode of Doctor Who. or Left Turn, same show. In Big Bang I'd just want to replace Amy and run around with the Doctor and have a grand speech at my wedding where everyone thinks I'm nuts and never got over my imaginary friend. Also, her husband is adorable and it was the proper beginning of the Ponds. And in Left Turns I'd just want to be one of the people that didn't seem all that important when Donna Noble met them, but I wouldn't ever replace her, she was the most important woman in the universe.