This overlooked performance can be by a living actor or one who has passed on. Many great performances are overlooked because they are in niche films, or in films that are mediocre excepting the great performance(s). I think many great performances simply get forgotten over time with the great deluge of movies we have before us.
My nomination is Audrey Hepburn’s performance in “The Nun’s Story”. She was great. The performance has been overlooked I think because folks focus on her work in her more well-loved movies.