This is about as simple as it gets. Three ingredients, nice and clean. That means use a quality rum and fresh lime. Always use fresh citrus. Keep in mind that agricole rhum (notice the spelling) is made with the juice of the sugar cane rather than molasses, so it's a bit lighter and grassier and has a bit of funk to it. Try tasting it at a bar first to see if it's your cup of... well, rum.
1.75 oz Rhum Clement white agricole rhum
.75 oz Giffard banana liqueur*
.75 oz fresh lime juice
Shake with ice, strain into a double rocks glass with fresh ice and garnish with a lime wheel or large twist of lime.
*I've tried a few banana booze products and this one is the only one you should be using. Real bananas were macerated in alcohol to make this, not some weird flavoring. If you see something called "99 Bananas" at the liquor store then you should run the fuck away.