Tough question. Laughter and empathy and loyalty and honesty go a long way. Loyalty in the sense of watching out for best interests. It's such a loaded word. I don't care for its use but it boils down to that's the reality when analyzing my most long standing friendships.

I don't consider myself the most perfect friend because I can be withdrawn at various points in my life. At the same time, I can hold secrets and I can remember the most seemingly innocuous details of what my friends like or who they are at their deepest core.
Oh and listening and thinking on advice or platitudes that suit the moment. I know when to zip it and not force my values onto them.