The Wild Bunch. Lost count of the times I've seen this and it still seems fresh. It's the story of a group of ageing outlaws in the early part of the 20th Century who come to realise that the days of the old west are no more "We've got to start thinking beyond our guns, those days are closin' fast."
They escape a posse by heading to Mexico where they take on one last job which has consequences for them all.
Great performances from William Holden, Ernest Borgnine, Warren Oates, Ben Johnson and Robert Ryan make, what would seem at first sight, deeply unpleasant characters, understandable and even noble in their own way.
Even by today's standards, it's still a violent movie - it was one of the first Hollywood movies to depict gunfights in slow motion and Sam Peckinpah doesn't pull any punches.
Certainly one of the greatest westerns ever made.