In my opinion, a well stocked bar would have any of the following, in approximate order of importance:
Wine: At least one red and one white, two or more varieties of each if possible so that you can a drier and a sweeter option.
At least one (preferably 2+) beer options. For basic starters, a Budweiser or similar, and perhaps a Sam Adams variety; that covers your basic standard, and something with a bit more flavor.
For basic alcohols, you'll want a mid grade vodka (as suggested by a PP), and a whiskey, blended scotch, or bourbon. I'd also go for a white rum. I'd put gin and tequila as secondary because most "martini" drinks nowadays are vodka based anyway, and tequila is typically only used in margaritas. (But keep this in mind for later expansion)
MIXERS. This is essential. Unless you're talking about a sipping drink like a single malt or a finer tequila, you won't get many takers for the hard alcohol that don't involve some mixing. If you don't think you'll be using it frequently, the little 8oz cans of coke, sprite, and ginger ale are perfect for this, since the individual cans will keep their carbonation. You'll also probably want to have juices on hand... Orange and cranberry most commonly, pineapple somewhat less so. Having lemons and limes or at least the juices would also be good.
That's a good start. Over time, it's fun to expand a bit, for instance, buying a bottle or two at a time. I'd eventually add on a dark rum and/or spiced rum (e.g Captain morgan), perhaps add a port wine and/or a brandy/cognac option. Then, expand upon the various whiskey varietals: there's basic Tennesse whiskey (Jack Daniels), Kentucky Bourbon (Jim Beam), blended canadian (Seagrams 7 or Canadian Mist), Irish (Jameson), as well as blended (Johnnie Walker or Dewars) and single malt scotches. The last being perhaps the most diverse of all.
You may enjoy checking out some of the various types of flavored vodkas, though in my opinion I can take or leave them. Good drinks can be mixed/flavored from more generic spirits, but some people enjoy the unique flavors.
In terms of liqeurs/cordials, the first three for me would be Peach Schnapps, Kahlua, and Triple sec, because having them on hand quickly opens up a wide array of drink options. (Black and white russians, fuzzy navel, sex on the beach, among others) Then, an Irish Cream like bailey's. (Not only a great sipping drink, but also opens up things like mudslides) After that, expand over time on things you come across that you like, or that you find to be unique. Off the top of my head, a few favorites I'd recommend are Amaretto, Frangelico, Sambuca, Campari, perhaps your favorite flavor of schnapps for something different. Then there's all of the "Creme de" types of liqueurs, mostly with only limited usage, but you may find some favorite flavors.
Okay, that's a long-enough winded list of suggestions
