This sort of question is really just her wanting to know what you love about her and your relationship. It need not be unique as in she is the best at .... or the best looking etc, just what you find unique about her and her mannerisms/attitude/personality that you like. There must be things about her which you see like this for example a certain smile she does, how she knows what to say when you are upset to cheer you up, or the aspects of her personality that you admire about her, or her view on certain things in life. you say she is not the best looking girl (very harsh imo) but there will be aspects of her you find attractive, her eyes, legs, waist, smooth soft skin, whatever.
I hope this has been of some help, Ive just tried to explain why she has asked the question and the kind of thing she is looking for as Im sure if you are in a relationship with her then you will have certain things that you love about her and that is what she wants to know. She wants to know what makes her special to you and different from other gfs, or other people.