Bitcoin, definitely.
While having your own currency is cool, you are restricting transactions to your island's boundaries.
So if the island was good at producing and exporting a product, it would be easier to deal with a currency that is more widely adopted.
Plus, they would save on exchange fees
Trust is also a large consideration. Many of us are immediately on guard when we hear of a new alt because the amount of p&d scams and shitcoins going around. A libertarian island will have a difficult enough time because of its politics with most people without introducing more concerns.
