simple, what voice(actor, famous person,entity) would you like so much that you would pay to have your ghost sound like them. I personally picked Scarlett from the movie "Her".
Edit 1- ok we've had some good ones that have literally made me lol, the reoccurring ones seem to be John Cena, Snoop dogg and Arnold Schwarzenegger
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Morgan Freeman would be hilarious for a day but it would get old. After watching the movie Her I would actually be down for a Scarlett Johansson Ghost. The best option would be Stephen Merchant though.