#3. Jack Swagger
Jack Swagger was technically fired from the WWE (although the official wording is that he was released from his contract), but it was an outcome that he absolutely wanted so it definitely counts as willingly walking away from the company.
Swagger made an appearance on Chael Sonnen's podcast and announced that he had requested his release from the company and had started booking independent wrestling dates. Two days later, WWE released a statement saying his release hadn't been granted. But eventually Swagger and WWE parted ways!
Since leaving, Swagger or Jake Hager has been relatively successful; he's the reigning Lucha Underground World Champion and he recently made his Mixed Martial Arts debut with a decisive victory for Bellator that even saw the WWE themselves congratulate him.