My sister wrote to me yesterday to ask about adding liquor to store-bought egg nog. While I highly recommend making your own egg nog -- it just tastes so much better than the stuff from the store -- I realize a lot of people don't want to put in the time to do this.
In that case, you definitely want to add some spirits to your nog. Otherwise, the stuff is just nasty, with no redeeming alcoholic kick. But what kind to add?
Bourbon is the traditional accompaniment to egg nog, and it works well. Don't bother putting anything too expensive in it -- the subtleties of the taste will just get lost. So use something like Evan Williams Black Label and you'll be off and running. If you want to go a little more upscale, you could always use Buffalo Trace.
There are some people, however, who are put off by the flavor of bourbon. For them, I would recommend using brandy as the spirit in your egg nog. You could go with a VS Cognac, although probably even that is overkill. Buy a bottle of good, cheap French brandy -- Raynal VSOP is excellent and under $15 -- and your nog will be more than worth drinking.
Your third option is rum. I love rum, and I do sometimes have it in my egg nog, but I don't think it works as well as brandy. It makes a nice change of pace, but it wouldn't be my go-to selection. If you do want to use rum, I would recommend a gold/dark Jamaican variety. Appleton VX would be great, but Coruba or Myers would work in a pinch.
The other question is: how much to use? This really depends on you. I would recommend something around 2 ounces of spirit per cup of egg nog to start. (Basically, one shot per mug.) You can always add more if you like it a little stronger.