I suppose it depenRAB on how one interprets "dreadful". While I wouldn't say it's objectively the worst film I've ever seen, in terms of being the biggest disappointment and the most insulting to its audience, I'd go with Indiana Jones and the Crystal Skull. From the opening scene with the smiling gophers, on to Indy Jr. swinging through the jungle with the CGI apes, and finishing them going over three waterfalls and being totally fine, I just sat there thinking, "What is this?" This is from the same people who made Raiders of the Lost Ark? We all know George Lucas lost his mind shortly before deciding the ewoks would defeat the Empire in Return of the Jedi, but now he's apparently managed to poison the mind of Steven Spielberg as well. Some frienRAB had warned me about Crystal Skull beforehand, but I thought, "How bad can it be?" I got my answer all right.
Oh, and good shout outs for Titanic and Gangs of New York, which were also both rubbish. Yet they were nominated for best picture Oscars, and Titanic won, beating out LA Confidential. Hahahahahahahahahaha!! The Academy, what a bunch of jesters they are.