While you might be able to get away with cheap rum or vodka whiskey is one of those things that you need to buy quality. Get the best you can afford because it really does make a difference. Good, really good, whiskey has a smooth, velvety texture and a mellow, sweet flavor. Bad whiskey tastes rough and burns going down.
Be sure to use real grenadine, not the fake red stuff that is called grenadine sometimes. It should contain sugar and pomegranate.
Serve it with a cherry and a slice of lime in an on the rocks glass. Or get some blood oranges - they look black on the inside and the flavor is very orange-y and sweet.