What kind of booze do I use?

You mean what sort of spirit should you use in a fruitcake? It all depends on the cake really, since they can obviously vary in style quite a bit. But as long as the liquor is hard, you can’t go too wrong. I’d say for a lighter cake, one that’s more yellow and buttery, I’d go for a brandy of some description, maybe even a fruit brandy if you’re going to get all fancy with your fruits (and really, why not get all fancy with your fruits?). For a darker cake, chockablock with spices and molasses, whiskey is the way to go. For some reason I tend to like Scotch when there are a lot of nuts involved, and bourbon when the cake is fruit-heavy (probably because that’s what my parents put in their very fruity fruitcake). I’ve never had rye in a fruitcake, but I can imagine it, since rye works so well with sweet vermouth. Whatever you do, no gin. I’d also avoid vodka since it doesn’t add much aside from a pure alcoholic zing. Don’t spend a lot of money. Fruitcakes have an awful lot of flavors going in them as it is, and whatever spirit you add needs to be able to play well with others.