My collection has become quite large recently, so it might not be the absolute worst game I own at this point, but the one that always comes to mind is Earth Defense, aka The Earth Defend. It's an unlicensed Sega Genesis shoot-em-up that is poorly designed, and has just terrible everything. The music is totally forgettable, the graphics are boring and ultra repetitive, the gameplay is lacking, the power-up system is flawed, the enemies take way too many shots to take down in most cases, and the enemy wave design is also lazy. Everything about the game screams "amateurish". I wrote a review of this for the 1st "Review A Bad Game Day" event in 2012:

http://metalfroreview.blogspot.com/2...th-defend.html