There are four big names among the clear liquors that the well-stocked home bar can never be without: gin, rum, vodka and tequila.
These days, you can't turn around without accidentally knocking over a cocktail made with vodka -- martinis, White Russians, Bloody Marys and screwdrivers to name a few. Your supply of vodka will take the heaviest hit, so it helps to have a top-shelf brand for guests who know their vodka and a cheaper variety for mixing. Gin has a somewhat limited number of combinations in cocktails, but once it's paired with vermouth or tonic water, the variations become infinite.
Rum is a necessity for anything tropical. Everyone has a preference, so keep a dark and a light rum on hand. Tequila is a bit of a wild card. It's rarer in cocktails (aside from tequila sunrises and the all-important margarita) but it makes a popular shot. According to an old and very believable legend, tequila shots temporarily cure heartbreak.