Big Rigs: Over the Road Racing is often voted as being one of the worst games ever made. As noted on wikipedia, the game is criticized for "Nonexistent collision detection, trucks that accelerate infinitely in reverse and stop as soon as the down arrow key is released, players being able to go up hills at a 90-degree angle and a computer controlled player that doesn't move".
However, it is also mentioned that this game sold over 20,000 copies.
A link is provided to a downloadable (trial?) version of this game.