You can't just have "a" whisk(e)y, you will need, at minimum: a bourbon (Jim Beam, Wild Turkey, Maker's Mark, Knob Creek), a scotch (ask someone else), an irish (Jameson's, Bushmills, Tullamore Dew etc.), and a tennessee (there are only two. Jack Daniels, and 5 points to whoever names the other). You might want a Canadian (Crown Royal, Canadian Club, one type of Seagram's) in there as well. And don't get cheap gin (Poland Spring), go for Tanqueray or Beefeater. Also, rum is handy.
I almost forgot, get two tequilas. Cuervo for mixing, and something good like Patron, Don Julio, or the like.
If you keep a bottle of Vodka on hand, you will always be able to cater to a variety of tastes. Vodka serves as the basis for many great drinks. It mixes well with any type of juice or liquor. Whenever I expect company, I make sure there is plenty of Vodka that way I can make Cosmopolitans for the ladies - which also requires Cointreau, Lime Cordial and Cranberry Juice - and Vodka Martinis for the Gents - which requires Vermouth. Black Russians are also popular - which requires Kahlua. Or you can keep it simple and just mix it with Orange Juice or Soda, both refreshing drinks. I personally don't really drink the stuff but as you can see, Vodka is very versatile. If you want a nice Vodka for your personal enjoyment try Grey Goose, if you are mixing it for the masses just get any old brand.
It all comes down to personal taste but I think Gin would be a good second choice for keeping in your stock. Gin makes a better Martini in my opinion. It also mixes well with Tonic or Juice. Tanqueray No.10 is my choice for Gin.
Wine is also good to have on hand, but it can be a bit touch and go. Some people will only drink White others only Red. Beaujolais is pretty inoffensive and will suit most peoples tastes. If you want a desert wine find a Late Harvest Reisling, it will cost less than an Icewine and has a fuller taste.
If this is just for your personal stock go to a bar and try before you buy. There are plenty of great Scotches out there, same with Port and Cognac - but have yourself a glass before you buy a whole bottle. Just because someone recommends a drink, doesn't mean you will like it.
Absinthe is mentioned above, if you like anise this might be a good choice - that or Sambuca - it may also be difficult find depending on where you live. The only advice I can give you on selection is to stick to Western Europe when picking one. Central/Eastern European brands like Hills should be passed over.
That being said Central/Eastern Europe has plenty of its own unique finds. My favorite from the Czech is Becherovka - though most people I know won't go near it. The Hungarians have Unicum which is also an original. And if you ever get a chance to try some sljivovica from the former Yugoslavia - don't pass it by, it will put hair on your chest.
Hope that helps. In the end it just comes down to your personal tastes.
We have a 32 bottle wine rack stuffed with various white and red grapes, optics in our kitchen for Vodka, Rum and Whiskey, and always three or four bottles of Verve in the fridge. We don't actually drink that much, but it's nice to have it there if we decide we want to.
Oh yeah, we've got a big red and blue neon sign that says "BAR" too. We light that up when we're on the lash
i can't really stand vodka myself, but i'll be buying a bottle to keep on hand for guests. my bestest loves vodka, and she's been visiting a good bit.
my stash consists of:
~ a handle of Southern Comfort
~ Johnny Walker Black
~ Bacardi Gold
~ Captain Morgan
~ Seagram's 7
~ at least one red wine with a nifty/fun label
~ a random bottle of margarita mix that i will use someday...really.
i don't make many fussy mixed drinks...i'm an "on the rocks" kinda girl. the only mixing i do is one of the liquors with either a cola (the SoCo or the Captain) or a lemon-lime soda (the 7).