I don't think one needs to be the "best" at something, but the combination of things that a person is passionate about or values is what I think of when I think of an identity. It encompasses many, many things, and that combination makes a person unique. For example: I'm passionate about programming, neuroscience, and playing violin. I don't think there are that many people with that combo of major passions, but I'm almost certainly not the only one. But when you keep adding things in, you become more and more unique. Specific values, feelings towards specific people and ideas, likes and dislikes, weird things that make you laugh. All of these things, together, forms your identity.