[HEADING=1]THIS IS MY OPINION[/HEADING]
The Orange Box. Now, I'm sure it isn't the worst, but out of what I've played, it definitely is. (It's probably average if I could play every 360 game.) Half Life just bored me. I liked the first one, but I couldn't find the sequels entertaining, unless I ran around killing things with the gravity gun.
Portal was too easy for me, and, while the story aspects of it are quite interesting, it wouldn't have been enough for me to buy separately from the game, if I had the opportunity. It wasn't fun enough for me to truly say "That was good". It was more of a meh.
Finally, Team Fortress 2. Now, while I've played some PC versions of it, and it is pretty fun, the Xbox 360 version is terrible. There is no updates from Valve, and overall, it just isn't fun for the 360.
Although, as for games that suck because of mechanical issues, probably Alone in the Dark.