Brandy - Cardinal Mendoza is a great popular Spanish brandy. Should be just for sipping though; I don't know much about brandy, but I don't want to know anything other than this one! J&M would be an idea for a cheap selection to add.
Bourbon - Woodford Reserve will please anyone from the bourbon drinker to the occasional or inexperienced imbiber. Not too expensive either. Good old Wild Turkey is a must on a shelf too - mixing, sipping, whatever; it's good, inexpensive, recognizable bourbon. I would suggest Jim Beam if I didn't hate it.
Gin - Bombay Sapphire and Plymouth are excellent gins for most if not all gin cocktails. A bottle of Beefeater's is an icon on a bar shelf though if you want a 'generic' one.
Vodka - Not much experience here, but the Russians drink Russian Standard. Very decent and well-priced. Tito's and Chopin are great too. Stoli is cheap and good for mixing.
Flavored Vodka(s) - Really whatever works. Anything but Absolut.
Whiskey - Most people are easily satisfied with Crown Royal and Jack Daniel's. No bar should be without a good rye for Manhattans, Old Fashioneds, etc., and Rittenhouse Rye is an excellent, cheap rye. Wild Turkey makes a good rye at a fair price as well.
Scotch - Not a lot of experience here, but The Balvenie Doublewood is good for beginners and experienced drinkers in the single-malt area. Johnny Walker Black is a bar staple, really, and the Red would be good for mixing.
Rum (Have this one covered 
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Sherry - Don't know much about sherry. Go for Spanish.
Tequila - El Jimador and Herradura. The end
Patron seems to be pretty popular these days too, so that would be a crowd-pleaser. 1800 is good for mixing. I would never be seen with Cuervo or Sauza in my house, haha.