QUOTE (Big Country @ 8/18/09 2:59pm)

and I want to be able to be sure to provide my guests with whatever they would like.
This is I think where you might be making things difficult on yourself. "Whatever they would like" is a really tall order. IMO the best way to do a simple bar is to establish a few staples that you like, and that are good quality bottles.
Furd has the right of it. My main concern would be staying away from too man choices in a type of alcohol. Pick your "house" brands.
I don't mix much. I might toss an ice cube in my scotch or bourbon, but I don't have to, and outside of that I don;t like to foul up mu liquor with other ingredients.
Vodka: Pick one that you like. Absolut, Stoli, Kettle One, Belvedere, Chopin, Smirnoff, etc. There are tons. If I could only keep one it would probably be the Chopin( (for sipping), but that's on the pricey side. If I was taking a second on it would most likely be the Stoli (for mixing).
Rum: Rum has come a long way and can be very enticing because there are so many ways to drink it. Many people stick within the Bacardi Light, Myers Dark, Captain Morgan, and Parrot Bay/Malibu Rum. Chicks dig Malibu Rum. Unless you plan on drinking the rum straight, you can probably get by best with a bottle of Captain Morgan and a bottle of Myers Dark. Third consideration would be the coconut rum, and then if you really wanted to go bananas you could find a nice sipping rum.
Gin: Tanguery is a really good choice. Unless you are really a gin martini guy, there is no reason to stock more.
Tequila: There's a lot of tequila out there these days. If you don't mind spending the $, a good bottle of Cabo Wabo will do you fine for sipping and for mixing. If you are looking to save $, buy the Cuervo Gold for mixing.
Whiskey/Bourbon: If I were keeping one, it would probably be the Knob Creek. Good juice for the price point. If you are a mix it with coke kind of guy (*gack*), then perhaps you want some jack. A bottle of Crown Royal wouldn't hurt either.
Scotch: Scotch is tough. You have your Single Malt Snobs and then your wannabe scotch drinkers/ scotch and water crowd. Johnny Black will do for the second group. If you want to keep one single malt, I keep the Balvenie Doublewood on hand. It's not super expensive ($40-$45 per bottle), and it's smooth without a powerful taste. Most people will be happy to drink it if they are scotch drinkers. I have yet to hear someone that did not take to it.
Cognac/Brandy/Armanac: All similar stuff. If you plan to make mixers with Brandy, a bottle of EJ's will do fine. If you are planning on having a nice sipping bottle, IMO it's worth the money to grab a bottle of Remy or Henessey XO. Delemain is nice too, but in my area it's more expensive and IMO not noticibly better enough to justify the money.
Port: I would suggest keeping a couple of bottle of Taylor Fladgate 10 year old tawney port on the bar.
The rest of the stuff Furd mentioned are the "Must Haves". Ask at the liquor store what the best vermouth* they carry is. It makes all the difference in the martini, much more than the Vodka does.
A couple of things I forgot...
Irish Whiskey A bottle of Jamesons or Bushmill will be appreciated by most adult drinkers that try it. Very under rated stuff.
Coffee Mixers Furd mentioned Bailey and Kahlua, but other things to consider are Sambvca, Frangelico and Amaretto DI Saronno (The only kind of Amaretto)
*Martini and Rossi will do, but Vya Extra Dry and Vya Sweet are top notch