I've walked out of movies, and I've also sat through some stinkers (mainly when I was a teenager) that were so bad I've forgotten them. So I can't really say what the worst movie I ever saw was. Of the better-known movies that I can remember, Independence Day was one of the worst. It was an action picture with no action. It was a blockbuster that was simply . . . boring. The alien spaceship (or whatever it was) lands on the freeway, and everyone gets out of their cars, stands there, and screams. The spaceship makes the road shake. Everybody stands by their cars and screams. Road shakes. Big noise. Screams. The hero saves the day by . . . typing on a keyboard (to do something completely stupid from a technical POV). What a snooze. The only reason I watched the whole thing was because I was on a plane and couldn't escape.
Another worst was Hannah and Her Sisters. Phony and dishonest. I decided afterward that I would never watch another Woody Allen film. I can't stand him.