Well, let's see.
You'll be wanting gin, bourbon, vodka, scotch, tequila, brandy, rum, and a couple of liqueurs. (All prices cited are for 750ml bottles and current as of today, unless otherwise specified. Prices are only a rough guideline; the actual price may be higher or lower depending on your location and shopping acumen.)
Gin - Bombay Sapphire ($19) is popular, but I find it to be a bit harsh and prefer Hendrick's ($32) for drinking straight.
Bourbon - Bulleit or Woodford Reserve ($26).
Vodka - Chopin ($30) for sipping and Stolichnaya ($19) for mixing. You might also consider picking up something really cheap if you have friends that won't notice, e.g. people that want to mix it with Red Bull.
Scotch - Well, there's a lot of variety here. Unless you're already a fan of Scotch Whisky, I'd suggest going with a blend if you're buying an entire bottle blind. The best way to decide would be to find someone (or some bar) in the area that has a good selection and start tasting your way through them.
Blended - Blended whiskies are made from a mix of malt and grain whiskies, and as a result are lighter and tend to have more wide-ranging appeal. The Famous Grouse ($19), Pig's Nose ($32), or Johnnie Walker Black ($27).
Vatted - Vatted whiskies are entirely malt whisky, but the whisky is sourced from more than one distillery. It's hard to go wrong with anything by Compass Box (~$45-$100). There's also Jon, Mark and Robbo's set of "The Rich and Spicy One", "The Smooth Sweeter One", and "The Smokey Peaty One" (~$30), which are as described.
Single Malts - Single Malts are entirely malt whisky distilled at a single distillery. It's a somewhat common misconception that it's also a single batch of whisky from that distillery--while there are single-barrel releases, standard expressions are blended from different barrels and ages in order to make the product more consistent from year to year. The age marked on a bottle (5yo, 12yo, etc.) indicates that the youngest whisky in it is at least that old. Scotland is divided up into various regions, and while the regions are geographically contiguous the division was done along stylistic lines. The four traditional regions are Lowland, Highland, Islay and Campbeltown; splitting up the Highland region into various subdivisions has become popular, but the only really important one is Speyside. Single malts attract snobs, but they're also quite tasty. Glenkinchie 10yo (Lowland, $42); Highland Park 15yo (Highland, $63); Caol Ila 12yo (Islay, $64); Lagavulin 16yo (Islay, $66); Springbank 10yo (Campbeltown, $65); Macallan 12yo (Speyside, $47); Talisker 10yo (Isle of Skye, $56).
Tequila - Don't like it. I'll still offer an opinion, though. Milagro Tequila Silver ($22) and Cabo Wabo Tequila Reposado ($36). Jose Cuervo Gold ($15) for people that just want to get trashed.
Brandy - Go with cognac, probably. Remy Martin Cognac VSOP ($34) or Planat Cognac VS ($20).
Rum - Tortuga Light Rum ($18) and Appleton Estate V/X ($20) or Pyrat XO Reserve ($23).
Liqueurs - Knock yourself out, as liqueurs are always fun. You'll want to look into making your own (especially easy ones like limoncello), but there are some (particularly herbal liqueurs) that are difficult to duplicate. You'll definitely want a triple sec, curacao, or other orange-flavoured liqueur; I like to keep both Cointreau ($32) and Grand Marnier Cordon Rouge ($35) on hand. Also popular are whisky-based cream liqueurs such as Baileys ($16). I'd also have at least one herbal liqueur, but why not pass up the macho frat-boy favourite Jaegermeister ($17) and get the less over-marketed Bénédictine ($32).
Bitters - An important ingredient to have on hand for cocktails. These usually come in small bottles and have a decent shelf life, so if you're serious about cocktails go ahead and pick up whatever ones you find.
As you look to expand, there are a few more categories of whisky that you should definitely get.
Rye - Sazerac 6yo ($30)
Canadian (confusingly, this is also be called rye whiskey in some areas) - Forty Creek Barrel Select ($20)
Tennessee Whiskey - George Dickel #12 ($20)
Irish - Redbreast 12yo ($47) and Bushmills 16yo ($66). On the slightly less expensive side, Bushmills Black Bush ($34).
Rum and rum-like substances -
Cachaca - Beleza Pura ($29) or Leblon ($27)
Rhum Agricole - Neisson ($32)
Sipping rums - Sea Wynde ($40) and Barbancourt Estate Reserve ($39)
Brandies -
Armagnac - Chateau de Briat Hors d'Age ($70)
More cognac - Pierre Ferrand Ambre 10yo ($35)
Apple brandy - Try a French Calvados like Busnel Pays d'Auge Vieille Réserve VSOP ($38) or Coeur de Lion Pays d'Auge Fine ($45). Domestically, Laird's is the most readily available; be sure to get one labeled Apple Brandy, as the Apple Jack is mainly neutral spirits.
Grappa - Clear Creek Grappa Moscato ($39) or Aqua Perfecta Zinfandel Grappa ($37 for 375ml)
Absinthe (mentioned upthread) is getting a lot of exposure now that it's legally available in the US. The only currently available one that I've tried is Lucid Absinthe Superieure (France, $74), which was decent, and would serve as an introduction to the drink. I also have a bottle of St. George Absinthe Verte (USA, $72), which is the first domestically produced absinthe to be made available and has gotten a lot of good press, but I haven't had a chance to crack it open as yet. Kubler (Switzerland, $51 for 1 liter) and Le Tourment Vert (France, $61) are also currently available, with several more brands pending (including at least one more domestic).
I think that's sufficient opinion for one post.
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