EDIT: Did you know that we don't have the #6 in Canada? That's my excuse for skipping over #6 in the Top 10 Reformed Villains countdown! Special thanks to Brian for picking up on that oversight, #6 will be revealed tomorrow!
5. Anya - Anya got her start in the Buffyverse as Anyanka, a Vengeance Demon. Her power was stripped from her when her charm was destroyed, leaving her trapped in the form of a teenage girl. She reluctantly began to spend time with the Scooby Gang, probably because they were the only ones around who had some actual knowledge of the supernatural world around them. On the surface Anya is rude, abrupt, and insensitive, but in Season 5 we got a real glimpse behind the wall she hides herself behind. With the death of Buffy's mom, Joyce, Anya gets scolded for asking stupid questions. But Anya, having only been human for a couple of years, simply did not understand what was expected of her because she had never been taught. The old Anya wouldn't have cared what the humans thought of her, but by this point she was in a serious relationship with Xander and was a full part of the team. The best transitions of villains to heroes is gradual, and Anya's accepting her role with the Scooby's, and the group accepting her as well, was developed over time to viewer's delight.