Crucial Spices (in no particular order):
Red pepper flakes
Vanilla extract (OK, technically this is a flavoring rather than a spice, but still)
Now here's what I would add to the list (and please keep this in mind: Simple and quick is my motto—which doesn't preclude tasty and healthy):
Garlic salt–Sheer laziness on my part really, not wanting to deal with fresh garlic sometimes.
Sesame seeds–They can be thrown in, or on top of, so many things, and they contain calcium!
Some kind of premixed spice blend–I like Cajun seasoning.
Old Bay–I'm originally from Maryland; nuff said (though there is more to be said: Here's how to use Old Bay beyond blue crabs).
Turmeric–Really I just use this for its purported anti-inflammatory properties, mixing it into scrambled eggs, for example.
Of course, if you didn't want to go out and buy a ton of spices just in case you might need them (hey, spices are expensive!), and you maybe wanted a big corporation to do the thinking for you, you could try McCormick's new Recipe Inspirations, premeasured spices and a recipe to go along with ’em. Personally, I think the concept's a small stroke of genius. (And before you pillory me for that statement, see my earlier note about simple and quick)
So what do you keep in your kitchen?