"Papillon" is a tiring but highly entertaining film that I am rating higher than it deserves for a specific reason. It's not perfect; it has many flaws that have already been explained in other comments, one of the main one being that the story is so vast, a standard feature film can hardly do it justice (it would have worked better as a miniseries). However, I can forgive all that and more, because this film contains a scene that left such an impression on me, I am sure it will be in the back of my mind for as long as I live. I am referring of course to the scene where Papillon is sent to solitary confinement for two years. It's only 25 minutes long in real time, but it's one of the most gripping and harrowing sequences I have ever seen. Papillon is enclosed in a tiny box, five paces across, no windows, high ceiling, constantly monitored by guards, and is not allowed to make the slightest noise. Without revealing too much, his resolution to get through the two years without trouble goes wrong early on, and the warden does everything he can to push him into total insanity. The fantastic suspense that follows is unmatched by almost any psychological horror film.
McQueen starred in "An Enemy of the People" a few years later, supposedly to prove that he could act in a straight drama, but he didn't need to. His performance in these 25 minutes is wonderful as he transforms from a quiet, confident man to a mumbling maniac. The elaborate makeup certainly helps, but the desperation in his eyes and voice is all genuine. There is no need to doubt: the man could act.
It's not the only memorable scene; there are many, but it's the most impressive. It's helped by Schaffner's intense direction, injecting more and more tension into each scene until it reaches heart-attack levels in the confinement sequence. The film's dialogue is also quite intelligent and quotable, moreso than in most prison films. The whole thing is a bit of chore to get through, but the scenes of human will beating the odds make it all worth it. 9/10.