@steepshot would be the one imho, but only if Steemit.inc gave them a 2m delegation. They won't, because they don't want to make competition for APPICS. So, it will either be @dlive or another dapp that hasn't yet been released.
I can't shake the image of driving up to McDonalds and the cashier giving you a QR code and saying "please upvote 10% here" and you getting your order 5 minutes later.
I have thought about this many times before, hopefully someone makes a dapp that will make it possible and businesses start accepting it.