Many liqueurs can be used to top ice cream quite nicely, but many of them have strange herbal essences that put some people off when served as dessert. Start with your favorites in small quantities or try some of my suggestions below. A dash or sprinkling is enough to provide a nice flavor combination, sometimes more is also good.
Amaretto is kind of a classic for either chocolate or vanilla ice cream. I occasionally dash some lemoncello onto French vanilla or chocolate or orange sherbet for a nice, but strong flavor. A very small dash of Aquavit over a fruity ice cream like peach or strawberry makes an interesting flavor. Why have Campari on ice when you can have it on ice cream? This is one of the few where I enjoy a fair amount of liqueur pooled over and around the ice cream. The strong flavor of green Chartreuse complements ice cream, and the color is most enjoyable on vanilla or other light bodied flavors. Pernod is pretty overpowering but less so on ice cream. You'd probably want to limit it to flavors you don't mind missing out on.
Liqueurs aren't the only bar supplies that work well with ice cream.
While it isn't going to buzz you, Grenadine tops bland vanilla nicely.
You can surprise your friends with a beer float using good vanilla and your favorite chili beer. The combination displays interesting contrast. Similarly, an inch of Southern Comfort in a rocks glass with a scoop of French vanilla is a fun cocktail -- I suggest swirling the drink between sips.
My current favorite Ice Cream with a Buzz is a drizzle of Vesta Virgin Cranberry Liqueur over Breyers French vanilla. This is most excellent!