I don't think there is any "ONE" alone. It is a combination of things . They are different for everyone, the key, I believe, is that your "lists" contain the same values and their degree of importance.
Personally, I looked for kindness, compassion, humor, integrity, morals and honesty. At some point in every relationship, one or more of these may come into question. If you place extreme importance on say, a sense of humor, and your partner has little or none, well that could be a deal breaker for you. You know what I mean?