B-movie Ed Wood and low budget horror type stuff is an easy target as is Kung Fu shit, Brit lads films and video game adaptations.
So the worst mainstream high budget feature I think I've encountered is Attack of the Clones, although I couldn't bring myself to see the other Star Wars rehashes. The first film, not made by Peter Greenaway, I have walked out of.
Utterly witless drivel from Goerge Lucas, a man who can no longer direct traffic. Back of a fag packet dialogue and the young evil galactic dictator Darth Vader was a whining little shit, who sounds like he is sulking after not being allowed out to a Justin Beiber concert. Also an early performance from the charisma free Natalie Portman. And when Christopher Lee turns up as a villian called Count Douku I could take no more.