I'm very careful with my purchases to make sure I get something I know I like, so I don't end up with games I later dislike. The only one within recent memory that would fit into the "games I bought and disliked" category are Adventure Time: Hey Ice King! Why'd You Steal Out Garbage? But I can think of a game that, though I personally have never played it, I can easily say is the absolute worst thing I've seen this generation.
Final Fantasy XIII (and by extension its sequel XIII-2). The worst writing I've ever seen, in any medium. How in the world do you mess up writing this horribly? It's almost antithetical in how it botches up basic writing techniques. And it's linear as all get out. Why can you never leave the endless hallway until somewhere around 20 HOURS IN?!
Even the gameplay is uninteresting (at least, from what I've seen). Oh, have the computer play your companions while you mash the A button over and over? Boring. How about you give me some control and let me fight however I want to fight, like the older games did? That always worked there, my personal favorite being FFVI where every single character had different methods of attack and even different input methods to spice combat up, along with the broken magicite system that allowed me to go totally OP on everything in my path. That's awesome, let's get back to that.