Agreed on A Clockwork Orange (although maybe I'd appreciate now I'm older) and 2001: A Space Odyssey was basically just special effects porn. I watched it only last year and thought that the effects were still pretty amazing, considering it's over 50 years old! But overall, pretty boring all round. The opening and closing scenes went on way too long. The whole thing was just Kubrick being self-indulgent.
But for me, the absolute worst film I've seen is Boyhood (2015). It's just so boring. I think everyone got carried away because it's filmed over 12 years with the same actors, so you literally get to see the kid grow up. But nothing much interesting happens whilst he's growing up to make it worthwhile spending three hours of your life on it and, in the end, he becomes your stereotypical angst-y teenager. Honestly, turning the TV off and staring at it for three hours would've been more interesting.