For us it was Avatar, but at least both of us couldn't stand it. I couldn't focus due to the 3D glasses not sitting at the right distance in front of my real glasses, and my bespectacled girlfriend got sick about 20 minutes in. We left, got our money back, and never bothered with 3D again.
She loved the experience, so I sat there for three hours watching a blurry movie because the 3D glasses were awful and causing pain.
Never again.