I have a hard time picking a "worst," since I really liked pretty much every game I played this year, but if I had to pick one it would be the same as my pick for most disappointing: Mass Effect 3.
A trilogy and years of personal choices and journeys flushed down the drain by the extreme hubris of Casey Hudson and Mac Walters. It's no exaggeration when I say this is one of the most disappointing things that I have ever experienced. I cannot believe how badly they messed this up and just how much they disregarded everything that the series was building up. Nothing you did over the course of 3 games mattered. Nothing you did in the entire game building up your forces mattered. The way they just disregarded everything to shoe-horn in their complete nonsense is just baffling.