I'm surprised nobody has included "Cool Hand Luke" on their list. Paul Newman was absolutely superb in it.

Also, while not a "true" prison film, I'd list "One Flew Over the Cuckoo's Nest" here. Stephen King gets two listings from me, "The Green Mile" (one of my all-time favorite movies) and "The Shawshank Redemption".

I'll round off my half dozen with "The Great Escape" and "Midnight Express".