I think the Decade's worst game is Big Rigs Racing, but I've never played it.
The worst game I bought was probably Superman Returns: The Game on the PS2. I had such high hopes. I know the Superman games have been awful, but this one actually has potential, and that's what makes it sad. If you've never had the pleasure of playing this train wreck then allow me to illuminate you.
It's a Superman game (very, very) loosely based on the newest Superman movie. It seems to have been inspired by the brilliant Spiderman 2 The Movie: The Game which had it's flaws, but the free roaming aspect really made it shine as you felt like Spiderman. Superman Returns is also free roaming, and gives you a city to protect with Superman's awesome powers. On paper it sounded great, and I was hyped.
In reality it is about 1/4 of a mediocre game stretched over the whole game. There are chunks of a decent game in it such as a good opening sequence with a big boss battle in a space arena to introduce you to the combat techniques, and a part about defending the city from meteors to introduce you to flight controls.
Then robots attack the city and you have to chase them through the streets and destroy them, and then more robots, and more robots, and more robots, and more robots, then a small boss battle with the guy from the opening sequence, then more robots, then more robots, then a giant robot, then more robots, then more robots, then that other guy again, then more robots, then more robots, then the giant robot again, then more robots, then more robots, then Mongul and that guy again, then MONSTERS!, then more monsters, then more monsters, then more monsters (I think you see where this is going so I'll skip ahead), and then we reach the exiting climax where Lex Luthor (who has been supposedly plotting the entire game, but he never actually does anything until now) unleashed his weapon from the film which creates Tornadoes for some reason (even though it doesn't in the film), so you use your superpowers to BLOW AWAY THE TORNADOES, and then more tornadoes, and more tornadoes, until the final big boss finale of a BIG TORNADO!
You may say well aren't a lot of games just some bad guys, and then some more bad guys? Let me tell you this there are about 3 types of robot all fought with either Ice, Heat, or pummelling them. Then there are the same number of monsters that are basically the same but with different character models. Each fight is EXACTLY THE SAME and there are each done dozens, and dozens of times. The boss battles are actually quite good, and break up the monotony. That's the trouble; this game is 95% padding. Add to that the fact Metropolis is no where near as well designed as New York was in Spiderman 2, and the combat itself is dull even without the monotonous fights, and you have an awful game.
Do you know what's worse? Not only did I buy this crap, but I actually finished the damn thing (which takes ages as there is so much padding). On the other hand this is possibly the best Superman game ever made (which tells you a lot).