Liqueurs are nice to stock in your bar because they look good on a shelf, allow you to make more complicated cocktails and last forever. But not just any liqueur will do. The must-haves are coffee and orange liqueurs. A cosmopolitan without Cointreau or triple sec is only a humble vodka cranberry, and a blended margarita without these liqueurs is only one step above a snow cone. As for essential coffee liqueurs, you must stock Kahlua and Tia Maria if you plan on making White Russians.
Aside from coffee and orange, other common liqueurs include amaretto, Drambuie and cinnamon, peach and peppermint schnapps. These are all popular and mixologically useful, but ultimately, the liqueurs you choose are influenced by your guests' tastes. For instance, you may not get a lot of use out of a bottle of Midori unless you know someone with a weakness for melon. Of course, you can always look stocking liqueurs as a learning experience -- every bottle is a new cocktail to make.