It's not hard to pick a coin for Starbucks, of course that would be STEEM coin or STEEM dollar because us Steemians are in the social network and Starbucks is where people go to not just have a cup of coffee but to socialize :)), I'm sure they want to be part of a social coin like steem that can bring them more business...hehe
PS, I'll be sending them a message to accept Steem and you can all do as well, cmon guys! :))