As pointed out here on several postings, some actors never realize their potential because they're stereotyped, or sucked into the Hollywood system. A classic film from back in the 60s was The Hustler. It was the best thing that Piper Laurie ever did in her career, and I loved George C. Scott's performance too, but moreover, I thought it should what a wonderful dramatic actor that Jackie Gleason was. It's no surprise that great comedians can play drama equally well. Robin Williams has shown this in a few of his movies, and Carol Burnett in Friendly Fire. Maggie Smith was great in The Prime Of Miss Jean Brodie, but not enough great screen roles seem to come her way after that. I remember seeing Kim Stanley on television in an episode of Ben Casey way back when, and I always wondered why she didn't do more film work. Some of my favourite underated actors have already been mentioned, but I'll add Lee Marvin and Lee Remick as well.