Well, thankfully I haven't played very many bad game systems. The worst that I played was Marvel Superheroes. Fun game, overall, but very annoying that not all heroes are created equal. Hmm, bad guy has Incredible rank body armor. Hmm, my character sheet has nothing over Remarkable...nope, can't hurt him a bit. Add to that the fact that your characer would never, ever get any better. Maybe it was our own fault for trying to run actual campaigns of this game. But, overall, if that was the worst game I've ever played, I'm not doing too bad.
For games I haven't played, any game that you can get killed before you start play is already in the top rank of worst games. Except for Paranoia, of course. If you die in character creation in Paranoia, it's just good clean fun!
Now card games...boy, let me tell you about card games. The absolute biggest stinker had to be SimCity the card game. To play, you had to have a mayor and some kind of power plant. These weren't necessarily in the deck you bought, however. Not to worry! There are cheap paper copies in the rulebook! So you start laying out a bunch of cards to build a city, but what they do, how they interact, what makes for good placement...these are all things missing. In the end, me and my buddy, after an hour of placing these cards down, just gave up and rolled dice to see who won. Then there's Illuminati: New World Order. Great idea. Really cool cards. Unfortunately, about all you can do is look at them. In a three-player game, the fact that you have to add up how each and every card interacts with each and every card every time anybody tries to do anything...well, it becomes an accounting nightmare very quickly. I hear they fixed Rage. They would have had to. When me and my buddy got ourselves some cards and built some decks, it quickly became obvious that we could simply compare who had the biggest werewolf. That deck would be the winner 99% of the time if we even bothered to play.