Live free or Die Hard. It would have been a decent general action movie but it sucked as a Die Hard movie. Every single thing that made John McClane John McClane was removed, the abundance of swearing, the witty/sarcastic comebacks, the general jack-ass behavior and of course the prerequisite line "Yippie-kai-ay ************!". The unrated DVD with swearing dubbed in and CGI blood added was just an insult.
Terminator Salvation was alright, far from the best but not the worst of the set. T3 was, in my opinion, the worst of the Terminator movies.
However, the absolute worst movie I have ever tried to watch was Mega Shark vs Giant Octopus (that really is the title). That movie made no sense at all. The special effects looked decidedly "special" and the acting was horrendous.
Certain things during the movie were never explained, like why a giant shark (bigger than a Megladon) jumped over a thousand feet into the air (up into the clouds) to bite a plane in half mid-air, or why the same giant shark lunges up and bites a chunk off of the Golden Gate Bridge.