Can't go wrong with gin n tonic. Mmmmmmm.
Here's what I'd stock with, you can't go wrong with any of those:
Finlandia vodka (it's a crime to mix with anything more expensive)
Grey Goose or Belvedere (for sipping, shots & cosmopolitans)
Bacardi white and gold (you'll need both to mix different drinks)
Beefeater gin (I actually prefer it over Tanq but go with Bombay Sapphire if you can afford, good for martinis too)
Cazadores tequila (damn good for the price and a hell of a lot better than Cuervo, $30 1L @ Costco)
Hennessy VSOP (mix w/ coke, cognac > whiskey, I hate drinking VSOP straight though)
Hennessy or Remy XO (sip or shots, crazy to mix a $100 bottle)
I admit, I never got into whiskey.
Next you'll need mixers (don't need all, just pick and choose depending on what you like to mix):
Kahlua or coffee liqueur (mmm white russians, or mix w/ tequila & coffee)
Midori or melon liqueur (tokyo tea/green dinosaur, melon ball)
Vermouth (martinis)
Cointreau (margaritas & cosmopolitans)
Triple sec (long islands)
Bailey's (mudslides, blowjobs, and car bombs - need irish whiskey & a good stout for car bombs)
Amaretto (amaretto sours, BJs)
lime juice
lemon juice
orange juice
pineapple juice
cranberry juice
mango juice (mango + tequila is damn good, or mango + midori for the ladies)
coke
sprite
cream or whole milk
tonic
coffee
There, that should get you started. Make sure you keep the vodka and the gin in the freezer and have plenty of ice on hand.
And who's the idiot that makes a bloody mary in a blender? Just buy a damn shaker.