That's a hard question.
Unreal II is a game I always wanted to finish. Many attempts throughout the years, it just doesn't work.
Battlefield Bad Company 2. A cutscene every 2 seconds. The worst FPS experience I have ever had. I would have liked it without those cutscenes, probably.
Jedi Academy. I remember my distaste. Can't remember the details. It has to do with moral choices or something.
Far Cry and The Thing are not bad games, actually they are pretty good games for the most part. Far Cry gets unimaginable hard at end game and ruins everything, one would wonder if they have level scaling or something. The Thing promised dynamic team management, it was as linear as it gets. I watched my team's annihilation because they didn't board the elevator with me, they all died or transformed in key points making everything I did obsolete. This ruins otherwise an amazing game.
The single worst game that I have ever played and finished from end to end. I haven't finished it yet. I am reserving that title for
I've never kept playing a game I don't like. Modern Warfare 2 was pretty crappy, sure, but it wasn't painful to play, and for a rental it was good being able to finish it before it needed returning I guess.
Worst games I've played? Dead Space (PC), never played it on console, but the PC port has such shoddy mouse controls I had to turn it off pretty much instantly.
Dead Space is an amazing game, one of my favorites. I like the limited controls as it creates tension, I still fixed it though.
The fixes I followed were, disabling Vsync in game and forcing it in nvidia control panel with triple buffering. That fixed the mouse issues for me.
Here is a youtube video that might help for ATI users.
http://www.youtube.com/watch?v=tqWwqXmdjKk
I recommend the sequel Dead Space 2(which improves on aiming and other things) and movies too:
http://www.imdb.com/title/tt1267379/ (Watch it after chapter 7 in Dead Space)
http://www.imdb.com/title/tt1711366/ (Watch it before Dead Space 2)