Batman & Robin was the most cartoonishly bad. It justifiably is popularly considered the very worst superhero movie ever made.

X-Men 3 was the biggest disappointment, though probably not the worst on any objective criteria. The first two movies were actually pretty good for superhero movies at that time, especially X-2, and the third was just a horrific mess of absolutely terrible writing. I agree with WK that it probably ranks at as the most hurtful movie for comic book fans.

Green Lantern was also bad, but there was no one on the planet who expected it to be good.