Mine is definitely Street Racing Syndicate.
That's what happens when you buy a game just because there's an inappropriately (un?)dressed female on the cover
As for the worst game I've ever played... my vote goes to Codename: Outbreak. The fisrt Windows 3D shooter I ever played (previously I had only played Duke Nukem 3D, Doom, Heretic and the likes).
IMO it was actually a sort of good game. The idea of team-based gameplay was great, and I actually liked the RPG elements of it - your squad members would gain experience based on your achievements in the previous mission, and their stats (firing accuracy, running speed, ability to absorb live ammunition) would increase, and you had a real inventory where you could store fancy stuff - like medikits (not many other games allow you to pick up medical supplies). However, the game was just too damn buggy - the AI was dumb as hell (the AI-controlled guy could spend all his ammunition trying to shoot enemies through the walls...27KM away), you would injure yourself jumping off anything more than 2 meters above the ground yet you could fall off a tall building without dying, among many other glitches such as blown-up explosive barrels re-appearing after loading the game. In addition to this, the graphics were pretty much a fail, the game was way too short and multiplayer = moderate fail, and the way-strong slavic-accented speech of the "american" voice actors really got on my nerves at times. Also, it was ripping off Alien (and a cllection of other movies) so hard I was about to start crying...
EDIT: To extend my previous rant, the only useful items you could store in your inventory were medical supplies, and those things were so scarce (unless playing on easiest difficulty) I had to press quicksave and quickload about as many times as I fired bullets (which was a lot, since it was an FPS). Also, the game was crappily coded, it crashed frequently and was really laggy at times, even if the system specs
way exceeded recommended requirements.