For me: One specific thing a great friend does for me is call me on my BS. Even if it's hard to take honest criticism from strangers or enemies, from a friend it's mandatory. Being brutally honest without being brutal is what I expect from a true friend.
The thing about being a great friend is that you can't do it by yourself. There's really only one thing that makes someone a great friend: A great friendship. It's a mutual thing all the way.