Just to add a little bit on the Mary Sue thing:
The problem with these types of characters isnt necessarily the fact that they are better at something, or even special, but rather the reason behind this. Often, these types of characters are made because people creating them want to have someone special, who is above everyone else and is loved by everyone because of how amazing they are. They want their characters to be center of the story and end up causing everyone around them to dislike the character.