This is a rum old fashioned using simple syrup instead of sugar. Since the rum stands out in this drink, it is recommended to use something that would be good as a sipping rum. I typically use Zacapa 23, Appleton 12, or El Dorado 12, but others will do. These each have a different flavor profile that may cause you to adjust amounts of bitters and simple syrup. Since those two ingredients are stirred with the drink, you can start with less and add more if you need it.