The most recent one I can recall is Johnny Knox breaking his back last year. He had surgery near immediately after the game. As far as broken backs are concerned, he is pretty lucky as he can potentially play again.
But the worst one is this:
Rutgers DT Eric LeGrand ended up breaking his neck, causing complete paralysis from the neck down. He is unable to resume anything in his life due to that terrible injury. He did, however, fulfill his dream of making it into the NFL as the Tampa Bay Buccaneers drafted him as an honorary member and let him retire as an NFL pro.