I couldn't say what my worst of all time are, but one that springs to mind is Command and Conquer Red Alert 3. I didn't mind the game so much, had some fun with it (and it kept some of Red Alerts sillyness which was good), but what I really dislike is what direction it took the series as far as mechanics. The original Red Alert is pretty much what got me into gaming properly (instead of just casually playing on my uncles SNES). EA just sucked all that was C&C out of C&C, especially considering it followed the (imo) excellent C&C 3. After RA3, I didn't bother getting C&C 4.
More recetly, I got the Force Unleashed 2, while 1 wasn't a great game, I had a lot of fun with it (and the story wasn't too bad either), 2 just felt rushed, bland story and I actually got bored with it by the end (at least it came around quickly).
I wouldn't call them the worst I've ever played (I have no idea what I would say is), but they are two that certainly stand out to me.