I have more than 30 types of chile peppers, and not a day goes by that I don't use at least one of them. Or would I have to choose one?
Seriously though if I had to choose the spice I buy the most of it would have to be coriander seed, which I buy by the pound, or actually, 500g, in the Indian market. This is used in many of the cuisines I cook, and most of what I buy is the Indian variety - the oval, golden colored variety. The rest I use is the small, reddish, round variety, from Thailand or Sri Lanka - stronger flavored than the normal grayish, middle eatern variety. Cumin would probably be second, and is also used in many cuisines. As for herbs, basil would be my favorite - sweet and Thai basils - what made me start growing it in the off season. However, the herb I use the most would have to be cilantro, since I use it in so many different types of dishes.