Dark rum can be e.g. Plantation Barbados, Gosling’s Black Seal, or whatever discount version. While it makes for a better drink with good rums, getting the right ginger beer and real lemons makes the largest difference.
The ginger beer is super important. Find one that is somewhat sweet.
Procedure
Pour ice cubes, ginger beer, and rum in serving glass
Squeeze both quarters of lime in the serving glass. It’s important that the limes are completely drained. Use one quarter lime for garnish