The original SAW was marketed as an action movie with Danny Glover and Ken Leung as the two cops trying to save Cary Elwes and Leigh Whannell before time ran out. For starters, imagine my surprise when the action movie I came to the theater to see was replaced by a horror movie; add in an unnecessary amount of gore and poor writing; compound that with a villain the writers were somehow trying to make us think was the hero; and finish it off with the stupidest, most improbable, worst ending ever, that could yet guarantee some idiots defending it as a good ending just because it was the type of ending that doesn't happen often (the bad guy wins) and it equaled the worst movie I've ever seen. To find out they made five more and a video game depresses me because nothing about the first one was good. The bad guy won. Big deal! That's not enough to make five more movies where the bad guy keeps winning. And the story sucks. If you're killing people for no reason and you say "I have a reason" even though you don't, you're still killing people for no reason, and that's bad writing.
Second place goes to the 2003 version of The Texas Chainsaw Massacre with Jessica Biel. Ugh.