Most importantly, the ability to speak with honesty. That's especially important to someone you're close to. If you say one thing, then I strongly believe you should mean it.
Trust, honesty, agreeability and being able to laugh all rate highly in my mind. Injecting humor into a conversation always works.
Definitly though, honesty and trust are the two best qualities. If you don't mean it, don't say it. If someone isn't honest with you, then you can't trust them, and that's the road to nowhere.