There is a singular, central, transcendent, primary, essential word to describe the heart of God's very being, character, person, and heart.
Contrary to popular belief, it is not holiness.
Listening to conversations on the ground, in churches, and in various theological circles, one might (understandably) gather that the most important, most fundamental characteristic of the God of the Bible is that he is holy.
To be sure, God is holy. God's holiness is witnessed, praised, described, experienced, remembered, tasted, honored, worshiped, sung, prayed, and otherwise attested from beginning to end in the biblical witness. The story of Yahweh with Israel, then with the church through Jesus and the Spirit, is clearly and profoundly the story of the holy creator God -- the "Holy One of Israel" -- acting in, with, and for his people and his creation. Much of an entire book -- Leviticus -- contains what is called the Holiness Code. There are countless ways in which the holiness of God affects God's people; in which God's people somehow engage the holy God; in which God gives to God's people the means by which they might, somehow, approach him. That this holy God of the universe displays his holiness through the intimacy and relationship of covenant is one of the greatest and most glorious mysteries of the Bible's witness. We worship God because he is holy; we worship God in his holiness; we believe God's promise to make even us holy as he is holy.
"Holy" is not the word we use for God. To quote Yoda: "There is another."
And that word is love.
God is love. Those three words name a mystery, a reality, a truth so world-shattering that no other words can adequately improve upon them. God is love. Not "God is loving" or "God loves," though those statements are certainly true. God is love. The noun. God is equivalent to the noun, to the thing, to the ontic reality that is love. God's self is identified with love. When we say "love" we say "God," and vice versa. To see love is to see God. To love is to enact God. To be loved is to be touched by God. To live in love is to live in God. To know love is to know God. The mystery simply does not end. God is love.
Nowhere else in the Bible is God equated with a noun. God is holy, not holiness; just, not justice; wise, not wisdom; good, not goodness; faithful, not faithfulness; powerful, not power. God "is" adjectives, because adjectives describe what kind of God he is. So to say God is holy or just or wise is to say that God is "like" the attributes of holiness or justice or wisdom, that he possesses them. God acts justly or wisely; God's character is constituted by holiness and wisdom. But God, as God, is not equated with, not identified as indivisible from any of those things.