So many come to mind. But, one that scarred me was Midnight Express. An excellent movie about the real-life imprisonment of an American college student, Billy Hayes, in the early 1970s. He was sent to a Turkish prison for smuggling hashish. Originally he was given 4 years but just before his release, his sentence was revised to 30 years. He was truly in a hell on earth, immersed in non-stop trauma and torture. Even his dad (a US representative) couldn't help him and he was entirely on his own. Brad Davis was excellent as Billy, descending into madness and desperate to escape (the title is prison slang for making a prison break). Davis was a fine actor, sadly he died young.
I was underage when I saw it, a friend and I snuck in to see it and some parts have remained crystal clear. Two in particular; in one scene, Billy loses his composure in court when he's resentenced. His anguish and rage were palpable, it was devastating.
Of all the horrors he endured, the one that had my friend and I actually wrapped around each other was when he completely snapped and viciously attacked the prison informant who had framed one of his friends (there were 3 other Westerners). The man (John Hurt) was too frail to endure what was coming. We were scared witless.
Warning: NSFW, it's a full-on berserker breakdown and rough to watch, particularly its conclusion.