6. Monty Python and the Holy Grail (1975)
Now here's an anti-climactic ending that really works, and unsurprisingly, that's because it's a comedy, and they completely make a joke out of it. Everything appears to have built up to an epic battle between our intrepid Knights and the French, who are holed up inside a castle. If they can get inside and get the Grail, then their quest is complete, and the movie is over. We see the forces on both sides, they begin to storm towards each other, when...a police car - yes, you read that right - zooms into shot, and begins arresting King Arthur (Graham Chapman) and Lancelot (Terry Jones), while another officer raises his hand up to the camera, seemingly ending the film. It's such a bizarre break with reality but also a brilliant way to end a film that is, of course, a thorough farce from start to finish. Rumour has it - though this can't be substantiated - that budgetary constraints brought this ending about, and if that's true, it certainly stands among the most fortuitous last-minute changes to a film in history, because this result is far, far better than anything else one could have expected.