I can’t think of any reason to pick anyone else than Korchnoi. He not only qualified for two title matches, but faced one of the greatest World Champions at his peak while being 47 years old himself, and had 15.5-15.5 before the last game. The previous match against Karpov was also a title match in a way since Fischer quit, and also there he lost with the smallest possible margin.
No one else comes close to those achievements. Maybe Topalov is a good Candidate for second place on the list if he doesn’t count as World Champion.
Who is Chucky?