How are you defining 'original'? Do you mean one that wasn't a film or TV remake or a sequel/ prequel, book/ musical adaptation etc? If so, then the highest-profile one is probably Inception. However, after 100 odd years of cinema, there's no story left to tell that can be considered truly original, so do you mean a film that shows originality in what it tries to achieve? If so, I'd say Avatar. I didn't like it and the plot was from from original, but the method of making it and presenting it certainly was.
