There are certain elements to an anime that makes it good: animation, plot, characters, etc. However, no great anime can do without an equally awesome soundtrack. It sets the mood, foreshadows action -- it makes a scene feel right. Audio is, after all, half of the anime-watching experience, and as much as a bad VA makes you cringe, a bad soundtrack can ruin the entire anime.
To recognize the great OSTs of 2006:
(Depending on mei's luck with finding audio samples, mei might upload some later.)