A priority friend is the kind of friend you would drop all prior plans to hang out with. The best kind of best friend.
Junior: Oh man!!! I waited at the pub for an hour and you didn't show up?? What happened?
Mike: Sorry man! I got "the call" from Chris last night wanting to hang out. I drop all my plans when he calls. Life is just more fun with a friend like him. Chris is my priority friend.
Junior: Oh, that's ok then. I'd do the same if I'd got "the call" from Chris. He's worth it.