I would have to say Venom.
Although, I would have to say Spider-Man 3 as a whole is actually worse than X-Men 3 for the sole reason that it ret-conned the way Uncle Ben died by shoehorning the Sandman into that narrative. That was forced and an unnecessary inclusion.
There are whole bunch of other problems related to Spider-Man 3, but at least with X-Men 3, even if the viewer does not enjoy it as much as the previous entries it at least does not ruin the 2 previous entries that came before it. Spider-Man 3, on the otherhand obviously does, because Spider-Man's whole reason for becoming a superhero in the first place was due to Uncle Ben's death, and Sam Raimi (A director that I had the displeasure of meeting.) shouldn't have messed with that.