Team Fortress 2. I'm sorry, but that's just my opinion. It's not a BAD game, it just became so boring and repetitive for me after a while. I wouldn't call this a BAD game, not at all, just pretty damn boring after a while.
And now for the real worst game, I'd have to go with any Need for Speed made after Most Wanted.
Most Wanted was awesome, I loved it. Carbon was a big MEH but hey it wasn't really that bad. Then along game Pro Street and ruined it for me. But wait, there's something else, Need for Speed Under-fucking-cover. The graphics are exactly like Most Wanteds graphics, which is not necesseraly a bad thing but Most Wanted was released in 2004, and you'd think that by 2008 you'd at least update the graphics a bit.
The sun ALWAYS stays on your face, It's so damn hard to race with that.
Oh and don't even get me started on how hard is to actually drive your damn car.
All in all, it's shit. I'm never buying another NFS again. I really tough Undercover would be decent, but no, it was just plain shit.