Just as an aid for this thread, here's the definition of a Mary-Sue.

Mary Sue: A story in which a new character is so brilliant, witty, amazing, attractive and wonderful that one (or all) of the main canon characters completely falls head over heels for. Basically, this character tends to be a thinly disguised wannabe of the author themselves. This type of fic is usually associated with romantic stories in which the female author lives out her fantasy of being with a particular character through her story. However this designation has come to mean any character that is over-emphasized, whether in the fanon or canon world. When you see or hear something or someone called a "Mary Sue," it tends to be meant as an insult.