
Several contenders come to mind, the first of which is the dreaded Werewolf system (I can't remember what edition, and haven't played since then, sorry). I'm probably being a little biased, as I had a terrible gaming experience because of one player, but making a character seemed completely unbalanced: it was way too easy to twink things and create and uber-powerful damage monkey. Unless you were like me, didn't know what you were doing with a new system and was trying to balance things out to be fair and not create a starting level-douche that the other players would hate.
The 2nd contender goes to DnD 4.0. Never has a game ever been so confusingly made because common sense would dictate that you don't put a friggin' MMO on paper. Healing surges? Really? You lost me on having fun and being different and made all the classes the same and boring, WotC. End of story.
Last, but certainly not least, is "We Didn't Play-test This At All". Sure, a simple, lovely card game with cards that are always changing the rules on all the players. The game could end in 11 rounds, or a single card play. Absolutely no challenge, strategy or fun at all. Instead of playing along, just to see how interesting the game can get, someone decides to end all the fun in the first 5 minutes and declare themselves the winner.
Usually, I can find some jewel in any game I try that will make me want to try again. Sadly, there are too many glaring flaws for me to show any compassion or love for those I mentioned here. There are probably plenty of flaws in games that I do love to play, but they cannot trounce the one that keeps me playing: fun.

This post was part of Gamer Banter, a monthly video game discussion coordinated by Terry at Game Couch. If you’re interested in being part of this, please email him for details.
Other takes:
Game Couch: The Worst Game Ever
Silvercublogger: It's all about integrity
Yuki-Pedia: Love, thy name is rage quit
Master Kitty’s World: What were they thinking???
Zath: What's the worst game you've ever played?
SnipingMizzy: What is there to love?
The Game Fanatics: What is the worst game you've ever played?
Pioneer Project: Making the best of a bad situation
gunthera1_gamer: The game I could have loved!
The Average Gamer: Gamer Banter: Worst Game EVAR!
Extra Guy: Battle Blaze — 700 pounds of ‘don’t do it’