The Cuba Libre gets its bright tropical flavors from fresh squeezed lime juice that is nicely complimented by the Coca-Cola and some good rum.
Whatever you do though, don't call the Cuba Libre a Rum and Coke, which by the way is just rum and coke. The Cuba Libre is a more complete cocktail with an interesting, but overlooked history. If want to find out more read Why the Cuba Libre Hates Being Called a Rum and Coke.
To make a Cuba Libre squeeze the juice from half a lime into the bottom of a Collins glass, toss in the spent lime shell, and muddle the lime juice and shell together.
Fill the glass with ice, add in the rum, top off with Coca-Cola, and give everything a good stir.
Muddling the lime really matters in a Cuba Libré, in our testing there was a clear difference between the muddled and non muddled versions of the cocktail. What the muddling does is release the oils in the lime, which really round out the flavors in the drink.
We also tried making a version with Mexican Coca-Cola, but couldn't taste any difference compared to regular canned Coca-Cola.