Robb Stark
King in the North
Acting wise I enjoyed her performance the most in seasons 2 and 3, but 5 was probably her best acting, even if it isn't anywhere close to my favorite season. That season, as [MENTION=12670]AnthonyCordova[/MENTION] put so well, challenged her. The role of Buffy changed and her acting was forced to change with it. Dawn added a completely new dynamic she hadn't had to deal with before and the death of Joyce added a new dimension as well. Those forced the character to change.