I'd have to say Mass Effect 1. I tried to like it. I really did. But, it was just so goddamn boring to me. I barely knew what was going on and I had played for 10 hours and did not see an end in sight. I plan to give it another chance in the future, but I just could not get to like it.
Half Life 2 was also a bit of the same conundrum. A well made game, yes. I actually liked the gun fights and the physics gun. I liked slicing people in half by picking up razors and shooting them. But the vehicles sections were too many, too long. First there was that jet ski, and then I got stuck at the Dune Buggy part and just bailed. No direction arrow? I mean, I'm not stupid, but if you have a huge samey brown landscape, give me some aid to know where I am going. I'll give it another shot down the road some day.
Portal I did like, but I could never finish it. I didn't feel like I was accomplishing anything by looking up youtube videos for how to complete a level. Once again, I may give it another shot.
Also, World of Warcraft. I played WoW on morphine in the hospital after a major surgery, so that must have been a factor. I got to like level 12 and had no idea what to do. Plus, I don't really like paying fees for a game. I may play it again, but it is unlikely.
GTA IV was another disappointment. Don't get me wrong, it was fun and well made. I just couldn't get into the story and characters. Everyone with the exception of "Neeko Beelic" was a complete tool, especially his cousin Roman "Captain Tee-tees" Belic, and their annoying friend Brucie. After a while, I wondered why I kept playing. I always find myself going back to it, and maybe I'll beat it one day.