Although it probably depends on what you mean by "defining." The most popular answers are Black Saint and the Sinner Lady, Mingus Dynasty, and Mingus Ah Um. Mingus himself really liked to push Let My Children Hear Music toward the end of his life.

I go with Mingus Ah Um because it gives you all of the trademark Mingus-isms (in melody, harmony, interplay, rhythm, and style) with some of the most recognizable Mingus tunes. It doesn't aspire to the grandness of programmatic scale of Black Saint of Let My Children, but it articulates its awesomeness over a set series of tunes. Hence, with you mentioning Kind of Blue off the bat, this is (to me) the parallel Mingus album.

I've always felt that Mingus Mingus Mingus Mingus Mingus (aka Five Mingus) is a really good gateway album for Mingus without watering down the artistry at all.