Gosling's Black Seal Rum is almost certainly the most famous rum ever to come out of Bermuda, harkening back to its first production run in 1860. While not the most nuanced dark rum on the market, it certainly is a workhorse, and not a bad choice for a house bottle. Plus, since Gosling's owns the trademark on the drink name, you'll need it if you're going to make a real Dark 'n Stormy!
The "Black Seal" name comes from the WWI era, when British sailors would fill empty champagne bottles with Gosling's rum and then re-seal them with black wax. The seal (as in, the marine mammal) on every bottle is a playful homage to its origins.