Catwoman.
However, while I don't think BvS is the worst (though it's up there), I do think it probably did the most damage. It did to DC films exactly what B&R did to the Batman franchise alone, in the Golden Age of CBM's nevertheless. Not to mention the fact it's a film that's been hyped in one form or another for 70 years, so the end result feels somewhat worse due to the lead up. That and unlike B&R, there's currently very little means through which you can just laugh it off...
However, while I don't think BvS is the worst (though it's up there), I do think it probably did the most damage. It did to DC films exactly what B&R did to the Batman franchise alone, in the Golden Age of CBM's nevertheless. Not to mention the fact it's a film that's been hyped in one form or another for 70 years, so the end result feels somewhat worse due to the lead up. That and unlike B&R, there's currently very little means through which you can just laugh it off...