Refreshing thread, Bronx King! I would vote for "The Wild Women of Cellblock C," but you're probably not looking for those kinds of movies, huh? But, seriously, folks...
I liked An Innocent Man with Selleck, The Shawshank Redemption, and The Longest Yard. Granted, that latter flick wasn't a deadly serious prison movie, but I still liked it. Murder in the First, although not another 'strictly prison' movie, was good. In fact, it had the only decent acting I've ever seen out of Kevin Bacon and Christian Slater!