EVAH, is a subjective term, but whatever, I'll go with GI JOE: Rise of the Cobra
They had a good cast, a director who had produced questionably good work, a good budget (bigger than Transformers 2), a decent premise (Elite Squad must has to kill bad guys) and they had a good source material.
They produced rubbish 1 dimensional characters and a load of crappy generic CGI laden action.
That might not sound so bad, but for me it was the worst EVAH, they had so much good going for it, but I walked out of that just feeling, "What's the f******g point of cinema if they're only going to keep pumping this stuff out, maybe I should just give up on this so called art form."
If I were the Conan of cinema, GI Joe would be the guy who massacred my village.