For me it'd probably have to be Nemesis.
A premise that beats the dead horse over the nature-vs-nurture debate, with a very limp script, no explanation given about Worf or Wesley (though his appearance was brief he was still there in uniform), Data reverting back to his S1 persona, the invention of the Remans, plus many other little niggles.
My biggest issue with it is the death of Data. Him sacrificing himself is very fitting, after all he gives his life for his Captain, friend and 'father figure', which is very touching and shows that he is very much human. Then they go and fuck things up by having B-4 becoming Data 2.0, which cheapens his death and all he had become.