First of all, I have to agree with those here who say the only way to truly taste a liquor, any liquor is to sip it straight.
Diet Coke?

You gotta be kidding me. In the first place, it has a horrible industrial solvent flavor, due to the deadly aspartame. Nutrisweet is a horror story of pharmaceutical company manipulation and big money that's not pertinent to this discussion, but let's just say I persoannly would rather sip Mr. Clean. How can any rational human being claim to be able to taste anything over the chemistry-set flavor of Diet Coke? :
?
My very first real job was in a liquor distribution business, where I learned a lot about how to taste and rate liquors. The correct method for any liquor is to put it into a brandy snifter or small wine bulb at room temperature, give it 3 minutes or so to allow the vapors to collect, then take a small sip, drawing a puff of air across the tongue and then swishing the sip around the tongue before swallowing.
Even putting the liquor over ice distorts its flavor. With Flor de Cana and others mentioned here, the difference can be very noticeable.
That being said, when I have a beverage and am not in a taste-test, I drink my Centennario with a splash of club soda and a wedge of lime.
Using the "correct" tasting method, Ron Centennario stands up proudly against most of the other rums mentioned here; Lurker has a right to his own tastes, of course...but I think Centennario is a top-ten, top-shelf booze, particularly the Commemorativo (about $15.00/litro at any Mas por Menos or Automercado the last time I visited C.R.
If you don't think you're going to have time to get to/through the duty free on your way out of town, buy a bottle of the Commemorativo at any local supermarket. The folks back home will be suitably impressed.